The SIP server on the remote host allows the enumeration of users.
Description
The SIP server on the remote host appears to respond differently to registration requests for valid and invalid usernames. Using that fact, Nessus was able to enumerate some of the valid usernames.
Solution
Configure the SIP server to respond identically to valid and invalid usernames. This can be done in Asterisk, for example, by setting 'alwaysauthreject=yes' in sip.conf.