In short: SIP trunks are virtual phone lines that connect an IP phone system (PBX) to the public phone network over an internet connection. They replace ISDN and analogue lines, let you keep your numbers, and add flexibility such as failover to another site or to mobiles. They suit organisations with a supported PBX, including self-hosted 3CX, or Teams Direct Routing.
What SIP trunks are
A SIP trunk does the job ISDN30 or analogue lines used to do: it connects your phone system to the outside world. Instead of copper pairs from the exchange, calls travel over your internet connection using SIP (Session Initiation Protocol).
Capacity is measured in channels. Each channel carries one simultaneous external call, and you can add or remove channels without new physical lines. Numbers are no longer tied to a building, so the same number range can move with you or be shared across sites.
With SIP trunks you keep and manage your own phone system. If you would rather the provider runs the phone system too, a hosted phone system is the alternative. Our ISDN replacement options page compares the routes.
When SIP trunks are the right choice
- Your PBX is modern, supported and does what you need, and replacing it would waste a good investment
- You run your own IP-PBX, such as self-hosted or on-premise 3CX, or a Cisco call manager
- You need to move off ISDN or analogue lines before the PSTN and ISDN switch-off on 31 January 2027
- You want to connect Microsoft Teams to a carrier of your choice through Teams Direct Routing
- You have specialist systems (call recording, contact centre or alarm integrations) that depend on your current PBX
If your PBX is near end of life, compare the cost of new SIP-capable hardware against a hosted system before deciding. Our hosted vs on-premise comparison helps.
Compatibility checks before you switch
- Does the PBX support SIP? Some older systems need a software upgrade, a SIP licence or a gateway card.
- Is it still supported? An unsupported PBX exposed to the internet is a security risk. Check the vendor's support status and patch level.
- Is the trunk tested with your PBX? We check that the SIP trunk has a configuration for your platform, including codecs and registration or IP authentication.
- How many channels? Size from your busiest period; see how many SIP channels do you need?
- Is the connection ready? Check upload capacity, QoS and firewall settings, and switch off SIP ALG where advised. See hosted VoIP.
- What about analogue devices? Fax, door entry, lifts and alarms may need analogue adapters or separate services.
Numbers, caller ID and emergency calls
Number porting. Your geographic and non-geographic numbers can usually move to SIP trunks, including ISDN number ranges (DDI blocks). We manage the port with your current provider; read number porting for the process.
CLI presentation. SIP lets you choose the caller ID shown on outbound calls, for example a main number for everyone or a direct number per user. The number you present should be one you own or have permission to use, and we check your caller ID settings as part of the set-up.
Emergency calls. 999 and 112 calls must work from every extension. Each number needs the correct registered address so emergency services know where to go. Multi-site organisations should make sure each site presents a number registered to that site, and home workers should know how emergency calls from their softphone are handled.
Resilience and connectivity
SIP trunks can be more resilient than ISDN, because calls are not tied to one set of physical lines:
- Failover to another site. If one office loses its connection, calls to its numbers can route to another office.
- Divert to mobiles or a backup number automatically when the PBX cannot be reached.
- Dual connections. A leased line with broadband or 4G backup, or trunks from two data centres.
- Burst capacity. Some providers allow short-term extra channels for unusual peaks.
Busy sites often put SIP trunks on a leased line for dedicated, symmetric capacity.
SIP trunks into Microsoft Teams
Microsoft Teams Phone can use SIP trunks through Direct Routing: a certified Session Border Controller (SBC) sits between the trunk and Microsoft's cloud, run by you, an integrator or as a service. It lets you keep a preferred carrier and numbers, or run Teams alongside an on-site PBX. Users still need a Teams Phone licence.
Forever Group offers this as Forever Teams, and we also supply Call2Teams, which brings an existing SIP trunk or PBX into Teams without an SBC on site. For the other Teams routes, see Direct Routing vs Operator Connect.
Contact us with your PBX make, your numbers and your busiest call volumes, and we will quote SIP trunks that fit.
More on SIP trunks
Why choose your cloud phone system with Forever Systems
We are vendor-neutral. Forever Group supplies and supports several cloud phone platforms, so we recommend the one that fits how your organisation works rather than the one we happen to sell.
- Side-by-side comparison of hosted 3CX, Microsoft Teams Phone, Forever UC, Forever Teams and Webex Calling for your users
- Contact centre and call recording options matched to your call volumes and CRM
- Number porting, handsets and set-up handled for you, including the move off analogue and ISDN lines
- Connectivity checked so call quality is right from day one
- UK support for moves, adds, changes and faults
- For BT Cloud Voice, see our BT Business Partner site cloudvoice.cloud
We work with businesses, schools, charities and public sector organisations. We do not sell consumer phone services.
Want a vendor-neutral cloud phone system comparison and quote?
Tell Forever Systems what you need and we will come back to you by email with options, not a hard sell.
Contact usFrequently asked questions
What is a SIP trunk?
A SIP trunk is a virtual phone line that connects your phone system to the public phone network over an internet connection. It replaces ISDN or analogue lines, and each channel carries one simultaneous call.
How many SIP channels do I need?
One for each simultaneous external call at your busiest time, plus some headroom. Check call logs for peak concurrent calls rather than total calls per day. Our SIP channels guide shows how to size it.
Can I keep my numbers when I move to SIP trunks?
Usually, yes. Geographic and non-geographic numbers, including ISDN DDI ranges, can normally be ported to SIP trunks. We manage the port and can divert calls while it completes.
Do SIP trunks work with Microsoft Teams?
Yes, through Teams Direct Routing, which uses a certified Session Border Controller between the trunk and Microsoft Teams. Services such as Forever Teams and Call2Teams provide this without you running the SBC yourself.
Are SIP trunks the same as hosted VoIP?
No. With SIP trunks you keep and manage your own phone system; the trunks only replace its lines. With hosted VoIP the provider runs the phone system as well.