I'm testing SIP communicator for use with the Faunhofer Open IMS, and ran into username related problems. When I enter the user identity into the configuration dialog, firstname.lastname@example.org, it gets shortened to bob, and the realm set to domain.tld. The registration then fails, because the username field in the Authentication: header in the authenticated REGISTER message is set to bob instead of email@example.com. I tested this by setting the username to username+"@"+realm in SipSecurityManager.getAuthorizationHeader(), and the REGISTER then worked.
Hmmm ... I can live with changing the source to make it work with my installation, but I'm not sure how to resolve this generally. Maybe add another field in the server settings, automatically set to username, that can be changed to username@realm.