Are you using a surge protection device?
This is a device used between the wall jack and the product to protect
the product against electrical power passed through the phone lines.
These can cause some fax communication problems by degrading the
quality of the phone signal. If you are having problems sending or
receiving faxes and are using one of these devices, connect the product
directly to the phone jack on the wall to determine whether the problem
is with the surge protection device.