What is SIP trunk?
A SIP trunk is a virtual connection that carries phone calls between a business phone system and the public telephone network over an internet connection, replacing physical ISDN or analogue lines.
SIP (Session Initiation Protocol) is the signalling protocol that sets up, modifies and ends a call. A "trunk" is the telephony term for a bundle of lines, carried over from the era when it meant literal copper.
The practical difference from physical lines is elasticity. An ISDN30 gives exactly 30 concurrent calls and adding more means ordering hardware; a SIP trunk is sized by configuration, so capacity can follow a seasonal peak and go back down afterwards.
SIP trunking is often chosen by businesses that already own a working PBX. The handsets, extensions and call flows stay; only the connection to the outside world changes. That makes it the lower-disruption route compared with replacing the whole system.
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