To configure Biscom FAXCOM Server with T38Fax.com, follow these steps:
2) Set the file permissions for your callctrl.cfg to read-only so that it is not overwritten by Biscom FAXCOM when you restart the FAXCOM service.
3) Open Windows Registry to \HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432node\Biscom\FAPI for BT\Parameters
4) Double click on "CallingPartyNumber" and input your DID including the leading 1 (ie. 12125558888).
5) Edit the "CallingPartyIpAddress" and input "sip.t38fax.com" (without quotes!).
6) Close Windows Registry
7) Restart the Biscom FAXCOM service.