What is SIP?
SIP and SIP trunking explained — the modern replacement for ISDN lines.
What SIP means
SIP stands for Session Initiation Protocol. It’s the technology that sets up and manages voice calls over the internet. When people talk about “SIP trunks”, they mean internet-based phone lines that replace traditional ISDN lines feeding a phone system.
SIP is ideal if you have a capable phone system (PBX) you want to keep, but need to replace the old ISDN lines being switched off by 2027.
When SIP trunks make sense
If you’ve invested in an on-site PBX that still does the job, SIP trunks let you modernise the connection without replacing the whole system — cutting costs and getting you switch-off ready. If you don’t have a PBX, a fully hosted system is usually simpler.
Frequently asked questions
What is a SIP trunk?
An internet-based phone line that replaces ISDN, connecting your PBX to the phone network.
Is SIP the same as VoIP?
SIP is the protocol that makes VoIP calls work — closely related.
Will SIP work after the switch-off?
Yes — SIP runs over the internet, so it’s switch-off ready.