Radio Shack Modem I TRS-80 Owner's Manual page 6

If
your telephone hook-up looks
like this:
Connect
like this:
M
3
/
Adapter
Radio
Shack
Cat.
No.
279-360
(Refer
to
the
text)
~-7\
Modem
telephone cord
Figure
2.
Connection
to
Four-Prong
Jacks.