This is all a theoretical exercise...
One would assume part of this task is getting to/at the voip packets. Physical access to the wire - insert a hub, MAC flood the switch, etc. a man-in-the-middle attack may be useful - this depends on the voip protocol you intend to target - SIP or H.323. With SIP you can attempt to redirect the call to yourself as a redirect server or network router gateway. H.323 is different - you'll need to redirect the call at startup to yourself by being the gateway/gatekeeper.

The hard part (for me anyway) is re-programming the softphone to capture the voip control setup (SIP or H.323) and then play the RTP stream as the packets hit the interface yet not be part of the call - kinda a softphone "monitor" or bridged connection. Conference call function maybe?

Good Luck with your project.