SIP trunking works by using the internet to connect a business’s IP-based phone system (such as a PBX system) to the public switched telephone network (PSTN), allowing the business to make and receive calls using their internet connection instead of traditional phone lines. It uses the SIP protocol to initiate, maintain, modify and terminate sessions with one or more participants. It allows multiple communication sessions such as audio, video, messaging, to occur simultaneously and to be controlled by the IP-PBX.