Hi -
I also had this problem and have a back-door workaround, but it assumes you are running IMS on Windows XP.
There is a file called "hosts" in the WINDOWS\system32\drivers\etc directory. This contains a list of IP lookups that are checked first before attempting a network DNS lookup. I found that the IMS was attempting to use the address "stun.fwdnet.net" for STUN, so I added an entry to the "hosts" file to redirect this address to the local host 127.0.0.1 as follows:
127.0.0.1 stun.fwdnet.net
After restarting Windows (not sure this is strictly necessary, but it probably is) then the IMS when starting up no longer attempts STUN but simply performs a SIP REGISTER to the extension server IP as defined in the VOIP settings menu.
Hope this helps...