VoIP Call Flow
VoIP (Voice over Internet Protocol) is a way to make phone calls using the internet as the transmission method, rather than a landline.
You can make and receive calls from other VoIP users, mobile users, and people with ordinary landlines. The integration between PSTN (Public switched telephone network) and VoIP is complete and undetectable to callers.
Just like a modem converts digital signals from the PC into analogue (voice) traffic for transmission over a phone line, a VoIP-enabled phone or VoIP adaptor converts your voice into digital packets (using the SIP protocol) for transmission over the internet. If you make a call to another VoIP phone, the opposite process occurs at the other end, and your voice emerges intact from the ether. If you make a call to a mobile or landline, the gateway decodes the call and sends it on as an ordinary voice call to the PSTN.
The diagram shows in simple form how calls are routed. User A dials the number for user B. The VoIP adaptor logs onto the routing server, which looks up the destination and sets up the call.
Thereafter, the voice traffic passes directly over the internet between A and B. When A calls C, who has a conventional landline or mobile, the VoIP adaptor logs on as before. The routing server looks up the destination and finds it is a PSTN number. The call is routed via the PSTN gateway into the PSTN network as a normal call. Thereafter, traffic passes directly via the gateway.