SIP trunk
A SIP trunk is an internet-based phone connection that carries inbound and outbound business calls, replacing analogue or ISDN lines while keeping your existing numbers and, often, your current phone system.
A SIP trunk is the connection that carries your company's telephone calls over the internet instead of analogue or ISDN lines. SIP is the method used to start and end those calls. You keep the same business numbers. Concurrent calls are sold as channels: each channel is one conversation at a time, so twenty simultaneous calls need twenty channels, not twenty physical lines.
In the office, the SIP trunk sits between your phone system and the public telephone network, running over the business internet connection. Staff keep the handsets or calling software they already use. Incoming calls reach your numbers, then the phone system rings the right person or plays a menu. Voice shares that circuit with the rest of the office traffic. The line won't stay clear without spare capacity and call priority on the local network.
SIP trunks fail when the phone service and the internet service are treated as separate problems. A fault then becomes an argument about whose circuit dropped the call. Too few channels produce engaged tones while you're fully staffed. If power or the connection fails, the phones fail with it unless there's a backup path in place. Quality also collapses when voice sits on congested broadband, or when the old lines are cancelled before numbers have finished moving.
When it matters
- →Analogue or ISDN lines are still carrying the office phone system.
- →Callers hear engaged tones while staff are available to take calls.
- →Call quality falls whenever the office internet is busy.
- →The phone provider and internet provider disagree after every outage.
Related terms
SIP trunk: common questions
What is a SIP trunk?
A SIP trunk is a service that carries your business telephone calls over the internet rather than over analogue or ISDN lines from the exchange. It connects your phone system to the public telephone network using your existing numbers. You buy capacity as channels, which are concurrent calls, not physical copper. Staff can keep using current handsets if the phone system is compatible. Call quality then depends on the internet connection and how the office network treats voice traffic.
What is the difference between SIP trunking and VoIP?
VoIP is the general method of sending voice over an internet connection. A SIP trunk is one way of doing that: it's the link between a phone system, often one you already have in the building, and the public telephone network. A cloud phone system is also VoIP, but the system sits with the provider rather than on a box in your office. You can use SIP trunks to keep an existing system, or move to hosted calling without buying trunks separately.
Can I keep my existing numbers with a SIP trunk?
Existing geographic and non-geographic business numbers can usually move to a SIP trunk, subject to the current provider and a number port. The port has to complete before the old lines are taken out of service, or the numbers can drop. Direct dial numbers for individuals can move as well as the main switchboard number. The process needs a planned cutover so inbound callers are not left on a dead line. Confirm which numbers are in scope before anyone cancels the incumbent contract.
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