If your team already lives in Microsoft Teams, it makes sense to take phone calls there too. To do that, Teams needs a connection to the public telephone network, and SIP trunks are one of the main ways to provide it.
The timing matters for UK firms. The legacy copper network is due to be retired on 31 January 2027, and Openreach has said it will not be delayed again. Businesses still running ISDN lines alongside Teams have a good reason to rethink how voice is delivered.
This guide explains the routes available and what to ask a provider before you commit.
Three Ways to Add Calling to Teams
Microsoft Calling Plans. Microsoft supplies the numbers and call minutes. It’s the simplest option, but the least flexible, and suits basic needs in straightforward setups.
Operator Connect. You choose an approved telecoms operator, and the connection is set up through the Teams admin centre. Microsoft sets strict requirements for approved partners, including service level standards. The trade-off is less flexibility, since you’re tied to Microsoft-approved carriers and it suits complex requirements such as contact centres less well.
Direct Routing. You connect your own carrier’s SIP trunks to Teams through a Session Border Controller (SBC). This is where SIP trunking comes in, and it offers the most flexibility.
How Direct Routing Works
The SBC sits between Teams and your carrier. In plain terms, it translates and secures the signalling and media between your carrier’s SIP trunks and Microsoft 365, so inbound calls flow from the telephone network through the SBC to the right Teams user.
Direct Routing is also the route that copes best with older equipment. It supports analogue devices, fax machines, contact centres and existing PBX systems alongside Teams. That makes it a common choice for firms that can’t switch everything at once.
What You Need in Place
Microsoft’s planning guidance lists the essentials. You need a supported SBC, telephony trunks connected to it, a public IP address, an FQDN in a domain registered to your Microsoft 365 organisation, a public DNS entry and a trusted certificate. Your firewall must also allow the required signalling and media traffic.
On licensing, users need Teams Phone and Microsoft Teams licences assigned in Microsoft 365. Forgetting this is a classic cause of delayed projects, so check it before you speak to providers.
Who Looks After the SBC?
This is the most important question to settle.
You run it. You deploy a certified SBC, either on your premises or as a virtual machine in the cloud, and manage certificates, updates and monitoring. It gives you control, but needs real expertise.
Your provider runs it. Many UK providers offer a hosted phone or managed SBC, sometimes called SBC as a service. They handle the integration and day-to-day management, which is simpler for smaller IT teams.
A mix. You might keep an SBC on site for local analogue devices and use a hosted one for Teams users.
Ask providers to be specific about who owns what, especially when a fault could be in Teams, the SBC or the carrier.
What to Look For in a UK Provider
Proven Teams Experience
Ask how many Direct Routing customers they support and whether they can share a reference. A provider that has done it many times will anticipate the problems you haven’t thought of.
Resilience
A single SBC is a single point of failure. Ask about redundant SBCs, ideally in separate locations, and what happens to calls if one fails. Also ask about failover on the carrier side and whether calls can divert to mobiles if your connection drops.
Number Porting and Management
Check how existing numbers move across, realistic timescales and whether old lines stay live until the new service is confirmed. Ask how numbers are managed afterwards, because the process differs between Direct Routing and Operator Connect.
Call Quality and Network Advice
The team's voice depends on your internet connection as much as on the provider. A good provider will ask about your bandwidth, router settings and Wi-Fi before quoting, and may recommend a network check.
Security
Direct Routing uses encrypted connections, but SIP fraud, where criminals exploit weak configuration to generate costly calls, remains a risk. Ask about spending limits, alerts for unusual activity and how the SBC is protected.
Emergency Calls
Check how 999 calls are handled and how location information is managed, particularly for hybrid and remote staff. Get the answer in writing.
Support and Contract Terms
Look at UK support hours, response times and who takes ownership of a fault. Check minimum terms, notice periods and the cost of adding or removing users.
Common Pitfalls
Missing licences. Team Phone licences are required, so budget for them separately from the trunk.
Expired certificates. The SBC relies on a trusted certificate. If nobody owns renewals, calls can fail suddenly.
Firewall gaps. Blocked ports are a frequent cause of one-way audio or failed calls.
One SBC, no backup. It works until the day it doesn’t.
Forgetting non-phone devices. Alarms, lift lines, door entry systems and card machines may rely on old lines and need their own plan.
Blame ping-pong. Without clear ownership, faults get passed between Microsoft, the SBC vendor and the carrier.
A Realistic Example
Consider a 60-person engineering consultancy with offices in Manchester and Birmingham. Staff use Teams all day, but external calls still run through an ISDN-based PBX.
They choose Direct Routing with a provider-managed SBC. Numbers are ported in two stages, one office at a time, with the old lines kept live until each is confirmed. Reception keeps a few handsets on the old PBX during the transition, then moves to Teams. A second SBC location gives them failover, and calls divert to mobiles if either office loses connectivity.
Planning and testing take longer than expected, but the cutovers themselves are uneventful.
A Sensible Plan
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Audit your lines, numbers and devices.
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Confirm Team Phone licensing for the users who need it.
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Choose your route: Calling Plans, Operator Connect or Direct Routing.
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Decide who manages the SBC, and get responsibilities in writing.
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Check your network and firewall.
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Pilot with a small group before moving everyone.
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Port numbers in stages, keeping old lines live until confirmed.
Allow plenty of time. One vendor guide suggests setup typically takes two to four weeks, longer for multiple sites or complex routing. Wavetel Business is one example of a UK provider you might speak to about this, and any provider should be willing to put their plan, pricing and terms in writing.
Frequently Asked Questions
What is a SIP trunk for Microsoft Teams?
It’s a virtual phone line from a carrier, connected to Teams through an SBC using Direct Routing, so staff can make and receive external calls in Teams.
Do I need an SBC?
For Direct Routing, yes. You can run your own certified SBC or use one managed by your provider. Calling Plans and Operator Connect don’t need you to run one.
Can I keep my existing phone numbers?
In most cases, yes, through number porting. Timescales vary, so start early.
Is Direct Routing better than Operator Connect?
Neither is better for everyone. Direct Routing offers more flexibility and supports older equipment. Operator Connect is simpler but ties you to approved operators.
Do I still need Teams Phone licences?
Yes. Direct Routing users need Teams Phone and Microsoft Teams licences in Microsoft 365.
Conclusion
SIP trunks let Microsoft Teams make and receive external calls with the flexibility to choose your own carrier, keep older devices running and manage numbers your way. The price of that flexibility is more design work, especially around the SBC, licensing and ownership of faults.
The practical takeaway is to decide your route first, settle who manages the SBC, then ask providers detailed questions about experience, resilience, porting and support. Start well before January 2027 so there’s time to pilot and port numbers in stages.
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