What phone system or server are we talking about here (so I can move this to the appropriate forum)?
In the MiCollab/MBG server there is a tool called Teleworker Network Analyzer... Download and run that at the remote location, it will tell you if you all the appropriate ports are open, but my guess is that if you can receive a call then that isn't the issue, but it should be able to verify it so you can move on to something else.
When you say you "cannot get a dial tone" does that mean when you lift the receiver/press speaker you cannot get a tone, or when you press 9 (or whatever your ARS access code is)? If you continue dialing the number does it seem to connect but with no audio (ie, call your cell phone or similar number, does it ring)?
Out of curiosity, does this occur at other remote locations (if you have any)? Is the MBG in the DMZ or in server-gateway mode? What router/firewall is at the remote location?