Disclaimer: I'm lead developer on Oreka.

You can very easily record all RTP traffic on a given ethernet span to wav files on disk using a sniffing tool such as http://www.oreka.org/. Most people don't use encryption yet in the VoIP field. This will catch SIP, H.323 and Cisco Skinny traffic, i.e. most of the existing traffic except IAX (asterisk) type traffic.