What is SIP Trunk
Comparing with PRI, the SIP trunk is another form of transmiting voice and data. It uses SIP protocol and is very common in VoIP environment. SIP trunk is able to control voice, video and messaging applications. Technically, a local IPPBX can establish SIP trunks with overseas Internet telephony service providers (ITSPs) to enjoy their telephone services (for example, to receive and place calls using an oversea number provisioned by the ITSP). In this situation, the SIP traffic is carried over Internet.
To connect SIP trunk, IPPBX does not need a special telephony card. It needs an extra ethernet interface instead.
In Hong Kong, telephone service providers also offer SIP trunk (as an alternative to T1) with flexible pricing and scalable capacity. The sip traffic is usually carried over a private network between the provider and customer as illustrated.