From the VoIPong home page:
VoIPong is a utility which detects all Voice Over IP calls on a pipeline, and for those which are G711 encoded, dumps actual conversation to seperate wave files. It supports SIP, H323, Cisco’s Skinny Client Protocol, RTP and RTCP.
There’s a server daemon that sets your network adapter to promiscuous mode and watches for VoIP calls. Calls are automatically logged and the G.711 encoded conversations will be named by date and dumped in wav format to an output directory (sox is required to make this work). There’s also a handy monitoring tool that will allow you to see what conversations are currently being monitored.
I tried it out this evening and it really works, recording both ends of the conversation to a WAV file. This could be a really handy tool for recording podcasts from a VoIP client that doesn’t have a recording feature (Netmeeting, for example). You’ll probably need to tweak mic levels on both ends, or one end of the conversation will sound louder than the other.
Jason also provides some OS X-specific installation instructions over on his Hackzine post.