Numbers & trunks
SIP TrunkAlso known as SIP trunking
Definition
A SIP trunk is a virtual phone line that connects a PBX or phone system to a carrier, softswitch or gateway over IP using SIP. It replaces physical lines such as analogue or ISDN PRI circuits, carrying multiple simultaneous calls (channels) and usually bringing DIDs for inbound calls and DOD for outbound caller ID.
How a SIP trunk works
The PBX and the provider exchange SIP signalling and RTP media over the internet or a private link. Capacity is sold as concurrent channels rather than physical lines, and calls are billed per minute or bundled.
| Authentication | How it works | Best for |
|---|---|---|
| Registration | PBX sends REGISTER with username/password; provider sends calls to the registered address | Dynamic IPs, sites behind NAT |
| IP authentication | Provider accepts calls from, and sends calls to, allow-listed static IPs | Data centres, SBCs, high volume |
What comes with a trunk
- [DIDs](/glossary/did): phone numbers routed to the trunk for inbound calls.
- [DOD](/glossary/dod): outbound calling with a caller ID the provider allows you to present.
- [Codecs](/glossary/codec): usually G.711, often G.729.
- Security: TLS and SRTP where offered, plus IP allow-lists.
- Failover: alternative routes if the primary path is unreachable.
Our guide SIP trunks, DIDs and DOD explained goes deeper.
Trunks to mobile gateways
A GSM gateway looks to your PBX like another SIP trunk, so it can sit alongside carrier trunks with routing rules deciding which calls go to mobile lines. GSMCalls supports trunks over UDP, TCP or TLS with registration or IP auth, codec priority, ACLs, prefix routing and failover. See GSM gateway for FreePBX or for 3CX.
Frequently asked questions
How many calls can a SIP trunk carry?
As many concurrent channels as you buy and your bandwidth allows; roughly 87 kbit/s per direction per G.711 call.
What is the difference between a SIP trunk and a DID?
The trunk is the connection that carries calls; a DID is a phone number routed over that connection.
Do I need a static IP for a SIP trunk?
Only for IP authentication. Registration-based trunks work with dynamic IPs and behind NAT.
Can a SIP trunk fail over to mobile lines?
Yes. With a GSM gateway configured as a second trunk, the PBX can route outbound calls to mobile lines when the primary trunk is down.