SIP trunking (Session Initiation Protocol) is a method in which voice calls are delivered over an IP network. Commonly also referred to as VoIP, SIP trunking is used on a PBX system that is capable of negotiating a SIP connection. Traditionally voice lines (telephone lines) would be delivered over copper wires, making it more costly and less versatile.
However SIP trunking is delivered over a dedicated or a public IP network such as the internet. Depending on your provider, SIP trunking can be as reliable or even more reliable than a traditional phone line.
SIP trunking is becoming more and more common globally as it can provide features and versatility an analog line cannot provide.