Simple definition
A SIP trunk is an IP-based telephone connection between a company's phone system, such as 3CX, and a phone provider. It carries calls to and from public telephone numbers without a separate physical telephone line for every extension.
Technical definition
A SIP trunk uses SIP signaling to establish and manage sessions. Voice media typically uses RTP or SRTP depending on the setup. Providers may authenticate a trunk by SIP credentials, authorized public IP addresses or other supported mechanisms.
- Caller Public number
- Carrier Phone network
- SIP trunk Call connection
- 3CX IP PBX Routing rule
- Extension User or team
A DID directs an incoming call to a destination; the SIP trunk is not itself an extension.
- One trunk may carry several concurrent calls; capacity depends on the provider contract and system capabilities.
- Incoming public numbers are often identified as DIDs or DDIs. A single trunk can serve multiple numbers.
- Inbound routing sends a DID to an extension, IVR, ring group or queue.
- Outbound rules select the trunk, dialed number format and authorized caller ID, subject to carrier policies.
What is a SIP trunk used for?
A SIP trunk connects employees and office endpoints to the public telephone network. It can replace or complement older telephone services and allows a PBX to route calls to different teams or sites.
Practical example
An organization uses 3CX with a SIP provider. A customer dials the reception DID; the provider sends the call to the trunk, and 3CX routes it to reception. An outbound call uses a configured rule and a caller ID allowed by the provider.
Common issues
- Trunk offline or not registered: inspect provider reachability, credentials and allowed addresses.
- Outbound calls fail: review dial plan, outbound rules and provider restrictions.
- Inbound DID missing or routed incorrectly: check number formatting and inbound routing.
- One-way audio: inspect firewall, NAT, SDP media addresses and RTP traffic.
- Choppy or disconnected calls: examine packet loss, jitter, codec compatibility and call timers.
How to troubleshoot a SIP trunk
- Check trunk registration or availability in the PBX and note the exact failure time.
- Test an inbound and an outbound call separately, using representative phone numbers.
- Compare calling and called number formats with the provider's required formats.
- Check DNS, SIP signaling, applicable TLS, NAT and media ports against the PBX and provider documentation.
- Share necessary call traces with the provider only after redacting credentials and sensitive data.
SIP trunk vs phone extension
A SIP trunk links the PBX to an operator or another PBX, whereas an extension identifies an internal destination. The number of extensions is not necessarily the number of simultaneous external calls.