I have read a lot of papers about VoIP security issues. The main focus is on SIP protocol. You can find hundred of papers about securing SIP. Even though other signaling protocols like H.323 and IAX or even media protocols like Real-time Transport Protocol (RTP) have security problems that have been described in enter link description here, there is almost no paper about them. I want to start a beneficial work on VoIP security and I am confused about this.
It basically depends on signaling protocol, since it controls encryptions and so on.
For example, H.323 supports the using of SRTP, TLS, or even IPsec via its H.235 standard. While SIP uses other methods.
So I think before you start working on security of VoIP, you should go through a comparison between signaling protocols. Although I advise of using SIP for your researches.