There are different commercial and non-commercial VoIP-providers.
A customer can choose from standard and/or non-standard services (protocols).
VoIP comes within different administration protocols, such as H.323, SIP and IAX2.
Voice data exchange protocol depends on administration protocol,
for example SIP (Session Initiation Protocol) uses RTP (Internet real-time protocol).
The RTP control protocol (RTCP) is based on the periodic transmission of control packet
to all participants in the session, using the same distribution mechanism as the data packets,
based on separate port numbers with UDP.
These all makes VoIP very sensitive for sniffing.
"Cain & Abel sniffer" is able to capture these packages.
But not the IAX2 in thise case.
Consult an nondependent security adviser for further information.
