The largest difference between the two is that a fax tone answers the line when dialing a fax number.
Some fax machines have a phone that can be used to test the connectivity of the phone line or to make phone calls.
Phone and fax on one line
Some phone/fax lines are used as a regular phone and also have a fax attached to them.
If the receiving party answers the phone before the fax machine, the line works like a regular phone line. If the fax machine answers it prepares to receive a fax transmission.
Phone and fax on one line with different phone numbers
Other phone/fax lines have two (or more) phone numbers connected to one line with distinctive rings for each phone number.
Many modern fax machines (and fax software) can be programmed to respond to the different ring and answer only when their number rings. When either number is in use, however, the phone line will be busy. This technology is called DRPD (Distinctive Ring Pattern Recognition).