If one user is using PSTN Network and another user is using VOIP Network or Either VOIP to PSTN,the inter-networking between two technologies is necessary. That requires the translation between different protocols,this can be done by Signaling/Media gateways.
Basically PSTN Network uses the Signaling protocol called ISDN User Part (ISUP) and VOIP Network uses the Signaling protocol called Session Initiation Protocol (SIP).
Basically PSTN Network uses the Media
Comparison of ISUP and SIP Messages :
https://telconotes.wordpress.com/2013/09/01/calling-from-pstn-to-voip/
http://www.tutorialspoint.com/session_initiation_protocol/session_initiation_protocol_to_pstn.htm
