Unable To Fax With Microsoft Fax

I have this problem where I was not able to send or receive using Microsoft Fax (for WinXP and 2000). I'm using a GPRS modem which I have got working for other fax softwares like Symantec and MightyFax. I was able to establish a connection but was not able to send or receive. I can hear the ringing sound from the remote fax machine I was dialing but no fax was received there. Error message is "An error occurred while sending/receiving fax". For Win2000, I was only able to send but even then, the fax was not clear.