Fax machines have phones because they are connected to a phone line. The phone handset on the fax machine is a way to access the phone line.
A phone attached to a fax machine works the same way as a regular phone. You can dial calls out by pressing the keypad the same way as a regular phone. If the fax machine rings, you can pick up the handset to answer the call (this might result in the fax not receiving).
Frequently Asked Questions
Do all fax machines have phones?
No, many do not. Phones are associated with older fax machines or commercial grade fax machines.