VoIPER: VoIP Exploit Research Toolkit

Checkout VoIPER, a VoIP Exploit Research Toolkit that will help you alot with VoIP security auditing.
VoIPER is a VoIP security testing toolkit incorporating several VoIP fuzzers and auxilliary tools to assist the auditor. It can currently generate over 200,000 SIP tests and H.323/IAX modules are in development.