What model router are we talking about? What IOS version are you running? What is the part
number of the HWIC?
When you say the GSM network is down, does that mean the port is admin down or the interface is down?
If the router has truely hung then the only way to figure out what is really going on is to obtain k-traces in rommon. The procedure is described here:
http://tools.cisco.com/squish/9112A
Many times a router is reported as hung when a VTY cannot be reached. Could you access the console before the router was reloaded?