The number you entered with your fax will appear followed in parenthesis with the number Faximum actually dialled. The phone cord may be defective or the phone service cancelled. To test for these two possibilities, unplug the phone cord from the modem and plug it into a standard telephone. Attempt to dial the same fax number as you used for the fax and see if the call goes through and if you can clearly hear the sound of the fax machine answering. The phone cord may plugged into the wrong jack in the back of the modem.
Most fax modems have two jacks, one for the cord that goes to the wall and one for a telephone.
On some modems it is easy to plug the phone line into the jack intended for the telephone. This can cause strange and intermittent errors so care needs to be taken to ensure that the cable is plugged into the correct jack. Make sure that if the fax line is configured for tone dialling that you actually have a phone line that can handle tone dialling. To test this, connect a phone that generates tones to the phone line and attempt to place a call. Change the modem init string to turn the speaker and listen to hear what happens.