A practical minimum interface is TD, RD, SG and DTR, DSR.
DTR and DSR allows stuff to be port powered. RS-232 has been so screwed up from the original specification, so you never knew which was was up.
A way to look at the spec is:
DSR/DTR basically says the device has power
CTS/RTS is basically for hardware handshaking. RTS - can also be looked at as "answer the phone" and CTS - Phone has been answered
RI - the phone is ringing
SG - Signal ground
PG - Protective ground
The RTS/CTS and DSR/DTR functions were not implemented correctly in many devices.