A phone system can look perfectly healthy until the first busy Monday morning, when calls queue, remote staff lose audio quality or customers cannot get through. That is why knowing how to set up SIP trunks is not simply a technical exercise. It is a chance to design calling around the way your people and customers actually work.
SIP trunks connect your phone system to the public telephone network over an IP connection, replacing traditional ISDN or analogue phone lines. For UK businesses moving away from legacy services, they can provide flexible call capacity, better control over costs and a clearer route to cloud telephony, Microsoft Teams calling or a contact centre platform.
The best installation is planned before it is provisioned. Here is what that looks like in practice.
Start with the business case, not the channel count
A SIP trunk provides call channels, often called concurrent calls. One channel supports one call at a time, whether it is incoming or outgoing. It is tempting to match the number of channels to the number of employees, but that usually means paying for capacity that will sit unused.
Instead, look at your busiest periods. How many simultaneous customer calls do you handle? Do teams make outbound campaigns? Are reception, sales and customer service all likely to be on calls at the same time? Your call reporting, current bills and contact centre data should give a useful picture.
Allow room for growth, but keep it proportionate. A business with 40 users may need 12 to 20 concurrent call channels rather than 40. On the other hand, a customer-facing operation with seasonal peaks may need more headroom or a provider that can add capacity quickly. This is one of the practical advantages of SIP: capacity does not have to be tied to a physical line installation.
You should also agree what sits behind the trunks. They may connect to an on-premise PBX, a hosted telephony service, a session border controller (SBC), Microsoft Teams through a compatible voice configuration, or a contact centre application. The right route depends on the systems you have, the skills in your IT team and how much day-to-day management you want to retain.
Check whether your network is ready for voice
Voice traffic is sensitive to delay, packet loss and jitter. A connection that works well for email and web browsing is not automatically ready to carry business-critical calls.
Start with the internet connection at every site where calls will be handled. Check available upload and download bandwidth, but do not stop there. Ask about latency, packet loss and contention, particularly at busy times. A stable connection with sufficient capacity is more valuable than an impressive headline speed.
Voice should be prioritised using quality of service (QoS) rules. In many offices, this means separating voice traffic onto its own VLAN and ensuring large downloads, backups or guest Wi-Fi do not interfere with calls. Remote workers need consideration too. Their home broadband, Wi-Fi setup and headset can all affect call quality, so give them clear guidance and test real-world conditions rather than assuming every connection is equal.
Resilience matters as much as performance. Consider a secondary connection, such as an alternative broadband circuit or managed mobile connectivity, for sites that cannot afford to lose calling. At a minimum, decide where calls should be diverted if the office connection or phone platform becomes unavailable. That might be mobiles, another location or a temporary voicemail and call-back process.
Choose the right SIP authentication method
Your provider will normally use either registration-based authentication or IP authentication. With registration, the PBX or SBC signs in using credentials. With IP authentication, the provider permits calls from a specified public IP address.
Registration can suit distributed or cloud environments, while IP authentication can be a straightforward choice for a fixed-site PBX with a static IP address. Neither is automatically better. The decision should reflect your network design, security controls and failover plan.
If you use an SBC, configure it as the controlled boundary between your phone platform and the SIP provider. It can manage interoperability, encryption, call routing and security policies. For smaller deployments, a managed hosted platform may remove much of this administration, but make sure you still understand who is responsible for configuration, monitoring and incident response.
Configure the SIP trunk and call routing
Once the service is provisioned, your provider will supply the details needed to connect: registrar or proxy addresses where applicable, authentication details, supported codecs, channel capacity and number ranges. Enter these carefully in your PBX, SBC or hosted platform, following the supported configuration for that system.
The technical connection is only one part of the job. You also need a sensible dial plan. Define how users dial external numbers, how inbound calls reach reception, departments or queues, and what happens when a call is unanswered. Make sure international calling permissions and premium-rate restrictions reflect your company policy.
For businesses with several locations, SIP trunks can centralise calling while preserving local numbers. A caller in Manchester may still reach a Manchester number, even if the team answering is based elsewhere or working from home. This can improve continuity, but it needs clear ownership of routing rules so changes do not become a guessing game later.
Plan number porting without disrupting customers
If you want to keep existing business numbers, number porting needs early attention. Gather a complete list of numbers, confirm the current provider and account details, and identify which numbers are genuinely in use. Incorrect records are a common cause of porting delays.
Do not cancel the old service before the port is complete. In many cases, cancelling first can cause numbers to be lost or make recovery more difficult. Agree a port date, understand whether it is a full or partial port, and keep a temporary call-forwarding plan ready if needed.
Porting is also a good moment to tidy up your published contact details. Check your website, customer emails, invoices, directories and signage. If you are consolidating sites or changing the way calls are answered, tell customers what will improve for them rather than making them work out a new route to your team.
Secure your SIP service from day one
SIP services can be targeted for toll fraud, unauthorised registration attempts and denial-of-service activity. Security should be built into the deployment, not added after a problem.
Use strong, unique credentials where registration is used, limit access by IP address where possible and keep PBX or SBC software up to date. Disable unused international destinations, set sensible call-spend limits and configure alerts for unusual call volumes or out-of-hours activity. Review administrator access as carefully as you would for any other business-critical system.
Encryption is worth discussing too. SIP TLS can protect signalling, while SRTP can protect the audio stream. Whether these are appropriate will depend on the devices, PBX and provider involved, but they are particularly relevant when calls travel across public networks or support sensitive conversations.
Emergency calling must be treated separately from ordinary routing. Confirm how 999 and 112 calls are handled, what location information is associated with each service and how this works for hybrid workers. A phone number alone does not always tell emergency services where a caller is, especially when users are mobile or logged in from different places.
Test calls the way your business uses them
Before switching over, test more than a basic inbound and outbound call. Place calls to UK landlines and mobiles, test transfers, voicemail, hunt groups, queues, caller ID presentation and out-of-hours routing. If you use a contact centre, test screen pops, recordings, reporting and overflow routes. If Teams is part of the design, test calls from desktop, mobile and meeting-room devices.
Run tests under realistic load if you can. Check call quality while other network activity is taking place and test failover by simulating a connection issue. Record the results, the configuration used and the support contacts required during the go-live period.
A phased rollout can reduce risk for larger organisations. Move a pilot group first, learn from their experience, then migrate departments in manageable stages. A single switch-over may be appropriate for a small, simple site, but it depends on the impact of downtime and the number of moving parts.
Keep ownership clear after go-live
SIP trunks are flexible, but flexibility needs governance. Decide who can request new numbers, change call routes, add channels or alter international permissions. Review usage and costs regularly, especially after a merger, office move, recruitment drive or change in working patterns.
Good ongoing support should feel straightforward: a person who understands your setup, clear escalation when something goes wrong and practical advice when your business changes. At Bulb Tech, that means treating communications as an operational service, not a collection of disconnected phone lines.
The real measure of a SIP trunk installation is simple: when customers call, they reach the right people, wherever those people are working. Build around that outcome, and the technology has a clear job to do.
