[VOIPSEC] IAX2 entity auth and more

[derek macmurchy]

I have been doing some reading on voip security and i have a couple of questions i hope someone could help with:
1. i have heard differing opinions on which is the predomiment signalling protocol for voip; some say SIP, others various proprietry ones. Can anyone point me to solid reasearch on this ?
2.  I have been reading the ietf draft of the IAX2 protocol; particularly the MD5 based unilateral entity authentication mechanism. Does anyone know precisely what is concatinated with the challenge and, presumably, the shared secret before being hashed and sent to the server as the authentication response.
3. the srtp rfc (i do not know the number as i am writing this on a nokia e61 phone) was released in 2004. Again, is there any research on how quickly this standard is being incorporated into, new or existing, products/applications ?