It seams to me like this may be a case of user error, as forwarding a line does no involve any hardware on your end (at least not in the tri-state area).
This simply involves the customer to forward the company main number to an answering service; alternate location or what have you. Your main line rings once at the location but cannot b answered, it is re-routed through your provider Bell Atlantic, Bell South, Sprint) and a function of outside the telephone system. I can forward any phone to any number by pressing *73 and dialing the number HOWEVER-
If you are trying to do this on a DID line, you may have some problems OR-
You may be trying to use the line redirect feature instead of the call forward feature.
What steps are you taking and where are you sending the calls?