What is the name of the jack that connects a handset
to the phone? I thought it was RJ11, but it's not.

bob@coolgroups.com wrote:
(phone, modem, etc) to lines provided by the local telco. RJ-11 is your
common home phone jack, with only 2 wires, Tip and Ring.

The modular plug/jack for the handset therefore would not officially be
an RJ of any number. That said, it has been commonly but wrongly refered
to as RJ-9. The handset plug is more properly just a mini-modular 4 pin
4 conductor plug/jack AKA 4p4c

An RJ-11 would be 6p2c

see http://en.wikipedia.org/wiki/RJ-XX for more info

--reed