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Last edited 03 Sep 2026

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SIP Trunk Providers UK: A Practical Switching Guide

1 Introduction
2 Assessing requirements before changing provider
3 Testing and planning the migration
4 Telephone number porting and cutover
5 Operation after migration
6 Selecting a SIP trunk provider
7 Frequently asked questions
8 Related articles on Designing Buildings

[edit] Introduction

Session Initiation Protocol (SIP) trunking is a method of delivering voice and other real-time communications services over Internet Protocol (IP) networks. It can be used to connect a private branch exchange (PBX) or other business telephone system to a telecommunications provider, replacing or supplementing traditional telephone services such as ISDN.

The transition from traditional public switched telephone network (PSTN) services to digital, IP-based networks has made SIP trunking an important option for organisations operating business telephone systems. Changing SIP trunk provider can nevertheless require careful planning because telephone numbers, PBX configuration, network connectivity and emergency calling arrangements may all be affected.

[edit] Assessing requirements before changing provider

Before selecting a new SIP trunk provider, the existing telephone system and its requirements should be documented. This includes the number of concurrent calls required at peak periods, the type and configuration of the PBX or hosted telephone system, the telephone numbers in use, and any required services such as call recording, voicemail, call forwarding, conferencing or international calling.

The capacity required for a SIP trunk is generally determined by the number of concurrent calls rather than the total number of calls made over a period. Network capacity should also be assessed because each active call requires bandwidth, and voice quality can be affected by congestion, latency, jitter and packet loss.

The existing telephone-number arrangements should be identified before migration. If numbers are being transferred between providers, the porting process should be planned separately from the technical configuration of the SIP trunk. The ability to port numbers can depend on the number range, geographic allocation and the services associated with the numbers.

Compatibility between the new SIP service and the existing PBX should also be established. Relevant considerations include supported SIP standards, authentication methods, codecs, transport protocols, number presentation, DTMF signalling, registration requirements and any restrictions imposed by the PBX or network configuration.

[edit] Testing and planning the migration

Where possible, the new SIP service should be tested before the existing service is disconnected. A test deployment can be used to assess call quality, interoperability with the PBX, inbound and outbound calling, caller identification, number presentation, voicemail, call transfer and other functions relevant to the organisation.

Network performance should be assessed under representative operating conditions. Voice traffic is sensitive to delay, variation in packet arrival time (jitter) and packet loss. Quality of service mechanisms may be used on suitable networks to prioritise real-time voice traffic over less time-sensitive data.

Business continuity arrangements should also be considered. SIP services depend on network connectivity and, in many cases, electrical power at the customer premises. A loss of broadband connectivity, local network equipment or power can therefore interrupt telephone services. Resilience may involve diverse network connections, redundant equipment, alternative call routing or other forms of failover, depending on the required level of availability.

The migration should be scheduled for a period when disruption can be tolerated and should include clearly defined responsibilities for the existing provider, new provider, network administrator and telephone-system administrator. Configuration information and contact details should be available to those responsible for the cutover.

[edit] Telephone number porting and cutover

Telephone-number porting is normally coordinated between the gaining and losing communications providers. The process can take longer than the technical configuration of the SIP trunk, so the porting timetable should be established before a final migration date is selected.

Where the existing service permits it, the new SIP trunk can be configured and tested before numbers are transferred. This allows technical problems to be identified without immediately affecting the existing telephone service. During the final cutover, routing and PBX configuration may need to be changed so that calls use the new service.

The old service should not normally be disconnected immediately after the new service becomes operational. An agreed overlap period can provide additional protection while inbound and outbound calling, number presentation, emergency calling and other services are checked. The precise arrangements depend on the providers and services involved.

Particular attention should be given to emergency calling. Telephone systems using VoIP or SIP may provide access to emergency services, but the location information associated with emergency calls and the operation of the service during loss of power or network connectivity need to be understood and tested. The arrangements should comply with applicable UK telecommunications requirements.

[edit] Operation after migration

Once migration is complete, the telephone handsets and PBX functions may appear largely unchanged to users. The principal change is that voice traffic is carried using IP networks rather than the traditional circuit-switched infrastructure.

This makes the performance and availability of the organisation's network and internet connectivity more significant. Monitoring can be used to identify problems such as packet loss, excessive latency, jitter, registration failures and loss of connectivity. Fault diagnosis may require examination of both the local network and the provider's SIP infrastructure.

A change of SIP trunk provider does not necessarily require replacement of the PBX. Existing systems can often be retained if they are compatible with the new service. However, migration can provide an opportunity to assess whether an existing PBX remains appropriate or whether a hosted or cloud-based telephone system would better meet future requirements.

[edit] Selecting a SIP trunk provider

Providers can differ in the network architecture, geographic coverage, number ranges, interconnection arrangements, technical support, resilience and commercial terms they offer. Comparison should therefore consider the complete service rather than relying solely on the advertised cost per channel or call.

Relevant factors include the number of concurrent calls supported, geographic and non-geographic number availability, number porting arrangements, supported codecs and protocols, emergency calling arrangements, resilience, service-level commitments, technical support and any charges for setup, numbers, call recording or additional services.

The location of technical support is not in itself an indication of service quality. More important considerations are the provider's ability to diagnose faults, communicate effectively during an incident and meet the agreed service levels.

Contractual conditions should also be checked before migration. These may include minimum contract periods, termination provisions, number ownership or portability arrangements, charges for unused services and requirements for returning or retaining equipment.

[edit] Frequently asked questions

[edit] How long does a SIP trunk migration take?

The timescale varies according to the complexity of the telephone system, number-porting requirements, network configuration and provider processes. A technically simple migration may be completed relatively quickly, but number porting and testing can extend the overall process.

[edit] Can an existing telephone service and a new SIP trunk operate at the same time?

In many cases, yes. Running the services in parallel during testing and the initial migration period can reduce the risk of disruption, although the precise arrangement depends on the PBX and the services being migrated.

[edit] Will staff need to be trained when changing SIP provider?

A change of SIP trunk provider may have little effect on ordinary telephone use if the existing handsets and PBX are retained. Training may nevertheless be required if the migration changes telephone-system features, call-handling procedures or user interfaces.

[edit] What are the main risks of a SIP trunk migration?

Potential sources of disruption include errors in PBX configuration, number-porting problems, inadequate network capacity, poor network performance, incorrect routing and failures affecting power or connectivity. These risks can be reduced through advance testing, documented configuration, appropriate resilience and a controlled cutover.

[edit] Does changing SIP provider require changing the PBX?

Not necessarily. A compatible existing PBX can often continue to be used with a different SIP trunk. Compatibility should be established before the migration because different services may have different technical and configuration requirements.