May I see your config file without your current IP's. This may be an issue with how the IAD is set up.
In short here is an example of a trunk group.
voice trunk T01 type sip
registrar primary server.domain.net
DNS or "name-server" including domain-proxy as well as domain-name are also important in resolving this issue.
Have you looked at your host settings? Do you have static or dynamic resolutions.