Architecture for bridging unwanted calls back to the PSTN (public switch telephone network) thereby freeing-up enterprise ports for other uses. When an incoming PSTN call is received into the enterprise via the PBX, a SIP server makes the decision to trigger bridging after obtaining the call reference and the interface on which the call exists. The SIP server sends a request to the PBX to trigger bridging. When the PBX determines it no longer needs to be involved, it sends the bridging request to the PSTN. More specifically, a mechanism is provided to trigger TBCT (Two B-Channel Transfer) from the SIP server thereby pushing call routing of unwanted calls out of the PBX and preserving PBX ports. The SIP server can retrieve the PSTN-related information from the PBX using either web services or a SIP-based protocol.