There are different types of pairing.
One is via bluetooth and only works if the phone and desktop are in close proximity
The one I think you mean is better called twinning. In this case, when a call rings the desktop, a second outbound call is initiated by the PBX (typically). The 2 calls in progress have nothing to do with each other and very little needs to be done on the local firewall (if SIP) to allow the external leg to answer the call. Once the call is established, there are 2 active legs, one inbound and one outbound that are conferenced together. If the trunking type is PRI, and more specifically NI-2 or compatible, there is an option for release link trunking where the PSTN can be signalled to conference the 2 channels at the central office and free up ALL channels to/from the PBX. When I last worked with SIP trunking, it did not have equivalent options for release link functions.
Things get more complicated when you start utilizing the Softphone capabilities as far as required hardware and software requirements are concerned but the softphone side of the call typically would utilize the internet connection for the voice network. This method only uses 1 channel for the inbound SIP/PRI. It is very important however with SIP that the gateway used for the SIP connection and the Gateway used for the internet connection be the same point or you will most definitely have 1 way or no way audio issues.
Thats the 5 mile overview. If you need more granular answers, tell us more specifics on the method your using, the hardware installed/available, the software levels of each component, the type of trunking and maybe even the vendor providing the trunking. You know, like everything.
