After investigating some more, I've noticed that HMP sometimes doesn't respond correctly with authentication information in some cases.
During boot, when it attempts to SIP REGISTER it receives a 401, and correctly re-sends the SIP REGISTER with Auth information and gets a 200 OK. This is correct.
When sending a SIP INVITE to initiate a call, it again receives a 401, but this time simply responds with an ACK and BYE, giving up. Why is this?
The realm in the WWW-Authenticate part of both 401 responses is the same ("asterisk"), so what is wrong?