Why would you need to reset the Sign-In address in Communicator?
Communicator automatically queries the AD for the correct Sign-In address when logging into communicator (if logged into the domain)
In case for some reason users have changed the Sign-In Address manually, Communicator does not query AD at startup.
If you want to reset this manual overridden behavior then you need to delete the following registry key.
If you want to do this for example in a login script then you can use this command line: Reg delete HKCU\Software\Microsoft\Shared\Ucclient /f
The next time Communicator starts up it will query AD again for the correct Sign-In Address for the current logged in user
Thanks to Marc J for pointing this out.