There are some software VOIP testing solutions vendors in the Raleigh NC (RTP) area. I will see what I can dig up or you related to the actual PBX test.
On the network front take a look at IxChariot (http://www.ixchariot.com). Out of the box it supports a number of VoIP protocol solutions for testing between endpoint agents in the network to determine call quality. For a performance testing tool it's also relatively inexpensive in the commercial space for the number of sessions it supports and (as a bonus) you can use it for testing your application data flows inside of a live network and determining exactly what is the network response time of an app independent of the scalability of the actual application under test.