Similar FAX receive problem with 6310/IrDA.
Phone rings, computer detects ringing and answers, but after approx. 1 minute the call is dropped. Fax service tells "receive failure" :-(.
Seems like a firmware bug?
Fax sending is OK.
do you have the possibility of sending faxes turned on with your GSM provider? If not this could be your problem. This sound you can often get when you don't have data transmission open on your SIM. That also goes for the FAX transmission.