What causes the terrible sound you hear when you dial into a fax machine number?

Answer 1

The sounds are produced for the machine to talk to the other machine similar to the way a telephone modem uses sounds to dial up and once connected uses a digital signal. Similarly your phone uses sound to make the connection to another line. if you had a recording of someone dialing the phone and played back into a phone receiver, it would dial the phone and no buttons would have to be touched. The phone system is based on sending and receiving sound signals ie dial tone and busy signal, 0-9, #,*, and many other sounds which not only let the person on the phone know what is happening it also is sending info to the system.