Christian M. 8 min read

What is number porting and how it works for businesses

Number porting is the process that allows businesses to retain their existing phone numbers when switching phone system providers, upgrading systems, or relocating.

This guide covers which UK numbers are eligible for porting, how the process works, common pitfalls, and what to do before, during, and after a port.

Contents:

What is business number porting?
What business phone numbers can be ported?
How business number porting works
What can delay, reject or break business number porting?
What happens on porting day?
What are the costs of number porting?
Number porting action checklist for businesses

What is business number porting?

Number porting is the process of transferring a business’s phone numbers from one provider or service to another.

It allows businesses to keep the same published, customer-facing phone number while permanently changing the underlying service.

While workarounds like “call forwarding” or “temporary redirection” are suitable for maintaining a phone number in the short term, they don’t work when fully migrating to a new phone system.

What are the benefits of maintaining a phone number?

Maintaining consistent phone numbers when migrating the phone service, provider or infrastructure has several benefits:

Customer trust and recognition: Clients and partners can continue reaching the business on familiar numbers, preserving the relationships and brand recognition built over time.
Avoids unnecessary work: It eliminates the need to reprint marketing materials, update online directories, or notify contacts of number changes, reducing both direct costs and administrative burden.
Geographic independence: Businesses can retain local or national numbers even when relocating, maintaining a consistent presence in key markets regardless of physical address.
Competitive agility: Retaining existing numbers means businesses are free to switch providers based on price, features or service quality, rather than staying with an underperforming supplier.

What is driving businesses to change phone services?

For many businesses, number porting is needed as a consequence of wider infrastructure changes, including:

The PSTN switch-off: Openreach is retiring the PSTN and ISDN infrastructure by January 2027. Any business still on legacy landlines will need to migrate to a business VoIP phone system and port their numbers in the process.
Moving to hosted VoIP or SIP trunks: Businesses upgrading their phone infrastructure need to bring their existing numbers with them to avoid disruption to customers.
Switching providers: Businesses moving to a more competitive business VoIP provider can retain their numbers through porting rather than starting fresh.
Office relocation: Businesses changing premises can port their geographic and non-geographic numbers to avoid losing their regular point of contact.

Read our VoIP explainer to understand why Voice over IP is why it is replacing legacy landlines.

What business phone numbers can be ported?

The portability of a UK business number is mainly determined by whether Ofcom‘s portability rules apply to it. Other factors include provider support, whether the number is live and working at the time of the request, and how many numbers are being moved at once.

Here is the portability of each type of UK business number:

Geographic numbers (01 and 02)

Protected by Ofcom’s portability rules.

Geographic numbers are tied to a specific area code (e.g., 0161 for Manchester; 020 for London) to signal local presence and trustworthiness, and are included in most standard call packages.

They are, however, not tied to a physical address, so a Manchester 0161 number can be ported if the business moves to a different area. Portability is determined by number type and provider support, not location. Lead times vary by provider.

National numbers (03)

Protected by Ofcom’s portability rules.

National numbers are location-independent, making them suited to businesses that want a single national contact number, and are included in most standard call packages.

Lead times vary by provider, but the port itself is straightforward.

Freephone numbers (0800 and 0808)

Protected by Ofcom’s portability rules.

Freephone numbers are free to call from both landlines and mobiles, with the business absorbing the cost. They are commonly used by customer service teams wanting to remove any contact friction.

As with other protected numbers, a provider cannot refuse a porting request, though lead times vary by provider.

Not protected by Ofcom’s portability rules.

Revenue-share numbers are higher-charge numbers where a portion of the call cost is returned to the business. They are increasingly infrequent, and cannot legally be used as the primary contact number for complaints.

Despite falling outside Ofcom’s protections, they are technically portable, so it is worth verifying on a provider-by-provider basis and confirming with both your current and prospective provider before relying on it.

Mobile numbers (07)

Protected by Ofcom’s portability rules.

Mobile numbers are standard 07 numbers, typically used by fully remote businesses or sole traders using a single device as their main point of contact.

They follow a separate porting process, handled via a Porting Authorisation Code (PAC). Providers are legally required to issue a PAC within two hours of a request.

Multi-number porting

Businesses running a multi-line phone system or hosted VoIP system often hold an entire DDI range (a block of phone numbers assigned to a single organisation) rather than a single line.

The same Ofcom rules apply by number type, so a protected range can be moved in full.

Expect more validation, paperwork and lead time than a single-number port. The whole range moves together, so it is worth confirming the timeline with your prospective provider before scheduling the switch.

How business number porting works

Number porting is typically straightforward for the business doing so because the gaining service provider (the one taking over the phone service) leads the process from start to finish.

Here is the step-by-step of what happens, including where action is required from the business:

1. Confirming portability and reviewing the current agreement

The business confirms its number type is portable and that the new provider supports it.

It is also worth reviewing the current contract at this stage, as early termination fees may apply if the existing agreement has not yet run its course.

2. Gathering account details and ensuring the account is migration-ready

Before completing any paperwork, the business retrieves its current account information from the existing provider, including:

Registered business name
Billing address
Account number
Any associated service details.

This is the best time to resolve any billing disputes, unpaid balances or existing port requests on the account, as any of these can cause a rejection at the validation stage.

3. Signing a Letter of Authority (LOA)

The business signs a document granting the gaining provider legal permission to act on its behalf and request the move.

The details on the LOA must match the old provider’s records exactly, as even minor discrepancies are a common cause of rejection. A rejection will lead to delays and potentially a small fee.

4. The gaining provider submits the port request

Once the LOA is signed, the business is out of the picture. The new provider contacts the losing provider and manages the request from here.

No further action is required from the business at this stage.

5. Validation by the losing provider

The current provider reviews and validates the submitted details. This is where the majority of delays and rejections occur.

6. A cutover date is agreed

Both providers agree on a date and time for the switch. These are the timelines under normal circumstances:

A single business landline or VoIP number typically takes around 10 to 14 working days from a valid request.
DDI ranges (multiple numbers) and more complex multi-line setups can take longer and may require additional checks.
Mobile numbers, handled via the PAC process, can be completed the next business day.

7. Cutover and testing

On the agreed date, the number moves to the new provider. Downtime is typically very short, often just a few minutes.

The business should test inbound routing, outbound CLI, extensions, voicemail and any auto attendants or call queues immediately. Issues should be raised with the gaining provider straight away, as the resolution window is short.

What can delay, reject or break business number porting?

Most arranged number ports complete without issue. Avoiding the following is key to preventing any delays or issues during cutover:

Mismatched LOA details: The most common reason for rejection is that the name, address and account details on the Letter of Authority do not match the old provider’s records exactly. This includes cases where a number is registered to a previous owner, former employee or parent company, and not the current account holder.
Unresolved billing disputes or unpaid balances: A losing provider cannot legally refuse a valid port, but an outstanding dispute gives them grounds to raise objections and stall the process.
An existing port is already in progress: If the number is already mid-port (e.g., from a previous failed attempt or a parallel request), the new request may be blocked until the first is resolved or cancelled.
DDI ranges split across underlying providers: When porting a block of numbers, some may unknowingly sit on different underlying providers or services (e.g., post-acquisition numbers). This can force the range into separate porting processes with different cutover dates.
SMS or fax services not declared in advance: If a number also carries an SMS or fax service and this isn’t flagged before cutover, the services are unlikely to migrate with its phone capabilities.
Old provider delays: Ofcom sets timeframes, but deliberate slow-walking does occur, particularly with high-value customers. Businesses have escalation rights if standard timelines aren’t being met.

What happens on porting day?

Porting day is the scheduled cutover where the number moves from the old provider to the new one. It is not instantaneous, as routing updates need to propagate across the network, similar to how DNS works for websites.

Ports are scheduled Monday to Thursday by default, usually mid-morning, though businesses can typically request a preferred time window when agreeing the cutover date.

During the propagation window, some calls may briefly show mixed behaviour. Some calls still reach the old destination while others land on the new service. This is normal and temporary.

Once the port is confirmed live, downtime is typically just a few minutes. Most customers will not notice at all.

Note that historic voicemails, call forwarding rules and block lists do not transfer automatically, and will need to be manually reconfigured on the new system after cutover.

What are the costs of number porting?

Number porting costs for businesses are not fixed. Ofcom only mandates that at the wholesale level (i.e., between communications providers), porting charges must be reasonable and proportionate to the actual costs involved.

Retail prices passed on to businesses are largely unregulated, meaning costs vary depending on the providers and services the business is moving between.

Here is the general picture:

Mobile number porting costs

Porting a mobile number is free. Ofcom’s PAC (Porting Authorisation Code) process means any business can switch networks without calling their old provider, they simply text PAC to 65075 and will receive their code within a minute.

The only costs to bear in mind are early termination charges if leaving a mobile contract before expiry.

Landline and VoIP number porting costs

There is no standard retail price for porting a landline or VoIP number. The process incurs a cost to both the old and new provider, which are passed down separately to customers as fees in various forms:

One-time porting fees: From the new provider, typically charged per line or number.
Early termination charges: From the current provider, separate from any porting fee.
Ongoing number hosting fees: Charged once the port completes.
LOA resubmission fees: If account details don’t match and the request is rejected, some providers may charge to reprocess, typically around £20.
Express or out-of-hours porting fees: If the transfer needs to be completed faster than standard timescales, providers may charge extra for fast-tracking the process.

Bundled number porting costs

Aside from mobile, which is free by default, number porting costs are often waived entirely depending on the circumstances of the switch. Common scenarios include:

Signing a longer contract: Providers will frequently waive porting fees for businesses committing to 24 or 36 month agreements, absorbing the cost as part of winning the deal.
Switching to a higher-tier service or bundle: Moving to a more comprehensive package, such as combining mobile, landline and full fibre business broadband under one telecoms provider, often sees porting fees dropped as an incentive.
New customer promotions: Some providers waive fees outright for new business customers regardless of contract length.

Bundling telecoms services in particular can reduce overall costs by 15-30% compared to managing them separately, meaning even where porting fees do apply, they are typically recovered quickly.

For a full breakdown of system fees, from installation and operation through to add-ons and ongoing charges, see our guide on business VoIP costs.

Typical all-in number porting costs

Based on the experience of our VoIP experts, businesses may expect to pay the following for number porting only (excluding other VoIP costs):

Scenario	Typical cost
Single number / line	£0 – £50
Up to 10 numbers / small PBX	£0 – £100
Blocks of 10+ numbers / complex multi-line	£0 – £200+
Mobile number	£0

Number porting action checklist for businesses

Use the following checklist to ensure your business is prepared before the port, knows what to test on cutover day, and what needs re-configuring afterwards:

Pre-port

The bulk of porting issues can be avoided with proper pre-port due diligence and preparation by doing the following:

Confirm your number type is portable and that the gaining provider supports it.
Review your current contract for early termination fees.
Retrieve account details from your current provider: business name, billing address, account number.
Resolve any billing disputes, unpaid balances or existing port requests on the account.
Flag any SMS or fax services tied to the number with the gaining provider.
Have the new phone system fully configured and tested before porting day.
Brief staff who handle inbound calls on the planned cutover date.
Set up a temporary call diversion to a mobile or alternative number as a fallback.
Check whether your business still runs FTTC or ADSL broadband, or alarm services that run over the same line (this is rare but exists). These need to be migrated or rearranged separately.

Porting day

Immediate testing during porting day is advisable as the window for resolving cutover issues is short. Have your implementation team be all hands on deck and ensure the following is done:

Reboot desk phones and mobile devices once the port is confirmed live.
Test inbound call routing to ensure calls are landing on the correct destination.
Test outbound CLI to ensure the correct number is displayed when making calls.
Test extension routing to ensure calls reach the right people and departments.
Test voicemail to ensure it is active, configured and accessible.
Test auto attendants, IVR and call queues to ensure menus and routing are functioning correctly.
Test fallback and out-of-hours routing.

Post-port

A few things will not transfer automatically and need to be reconfigured manually on the new system:

Re-apply any call forwarding or divert rules.
Re-enter block lists and spam filters.
Migrate SMS or fax services if not handled during cutover.
Confirm historic voicemails are saved as these will not transfer from the old provider.
Raise any routing issues with the new provider immediately as the resolution window is short.
Confirm cancellation of the old service with the previous provider, as porting does not automatically terminate the contract or stop billing.