DTR (Data Terminal Ready). It is pin 20 is the cause
of 75% of the trouble people have with their modems. The
computer places a low voltage on this pin whenever it wants your modem to hang up the
phone. Strangely, most modems refuse to pay attention to DTR
unless coaxed with special initialization commands or switch settings. Some ignore
DTR no matter what
you do. Those modems insist on a special ~~~ATH0 command
to hang them up that works about 50% of the time.