It looks like SIP has at least two places that use MD5 digest:
One of which is in the password storage on disk on the SIP server - that's the MD5(username:domain:password).

The other place is during the in-flight network operations, and that, I think, is what quentusrex was referring to, since he was mentioning the nonce/salt and two md5 operations.

SIP password storage from one vendor, under a1-hash
of which the most relevant part is:
openssl dgst -md5 < filename, or echo -n "username:domain:password" | openssl dgst -md5.

Additional guidance on the in-flight MD5 use may be found at:

I don't have FreeSwitch running, but when I have a test install, I'll generate some test samples.

Messages In This Thread
SIP (MD5) - by m-i-k-e-e - 08-13-2012, 08:51 AM
RE: SIP (MD5) - by atom - 08-13-2012, 10:41 AM
RE: SIP (MD5) - by ShArkY_ - 08-18-2012, 07:11 PM
RE: SIP (MD5) - by atom - 08-19-2012, 10:26 AM
RE: SIP (MD5) - by quentusrex - 08-31-2012, 07:59 PM
RE: SIP (MD5) - by Incisive - 08-31-2012, 09:13 PM