Right now the hashing algorithm is most commonly MD5, though SHA2 is now supported in HTTP and has a draft standard for use in SIP (edit: RFC 8760 says it's ready for SIP too), so in theory there may be weaknesses at some point but for now the multiple uses of MD5 plus the use of nonces defeat any of the shortcuts I'm aware of. The end result is that when implemented correctly it's impossible to retrieve a password from a SIP authentication capture by any means other than brute force. There can optionally also be a client-chosen nonce and a nonce counter to further detect replay attempts, and even a hash of the message body to provide integrity checking. This scheme hashes the username, realm, and password together, then does the same for the request method and URI, then hashes both of those together with a server-chosen nonce to prevent replays directed at the server. To expand on the other posts, SIP authentication uses a slightly modified version HTTP's Digest authentication scheme.
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