Hello please tell us the make & model of your fax machine?
Thank you. When you dial the number from the fax machine do you hear the ringing tone on the other end from the printer speaker and subsequent fax pickup?
do you have problems sending to any other fax machine ?
You can try dropping the baud rate down sometimes this is a mismatch between fax types. There are about a dozen different fax modes and usually 2 machines will negotiate to find the best one that they can both use but if the other fax machine is too old it might have some issues with it.
also make sure that it is set to fax in black and white mode. Not many pure fax machines can handle the color mode and that tends to kick it right out if you try.
under the fax settings on the printer
set the baud rate or tx start speed to 9600
set ecm to off
set color tx to automatically convert to black and white