Part of the GSM standard includes the ability to forward calls. Most handsets have the facility built-in to their menu structures (look for something like Menu > Settings > Calls > Call Forwarding) but if not there are standard GSM codes that can be used.
http://en.wikipedia.org/wiki/Call_forwarding has the details.
The forwarding is done from your networks central network and is unaffected by the local signal conditions of your handset.
Be aware that the cost of the diverted leg of each phone call is charged to your mobile phone bill.