How does SIP trunking work?
Architecturally, SIP is drastically different from your current ISDN technology. However, the principles of delivering a phone call remain the same.
Whereas ISDN circuits require physical installation into your premises, SIP trunks are configured and delivered over an internet connection. Rather than a BT Openreach engineer attending site, your network provider will configure a dedicated VLAN for SIP to be delivered. Using existing SIP interconnects, SIP traffic is then sent over your private network connection.
SIP can be delivered using the public internet. Usually this is seen when deploying hosted voice services and users need to communicate when working remotely. Softphones can be deployed on mobiles, laptops and tablets to achieve mobility – leveraging home Wi-Fi, guest Wi-Fi or mobile connectivity.
SIP trunks themselves setup the SIP session for voice traffic to be transmitted. Speech (or video or instant messages in some scenarios) are then transferred via the SIP session. The SIP trunk then terminates the call. Just like ISDN and PSTN lines, the SIP trunks simply provide the mechanism to transfer your voice traffic.