There are several similar-looking plugs used for telephone headsets. The situation can be very confusing, so be sure you get the right type of headset for your phone.
There are unimportant and important variations in the plugs.
Unimportant variations include the material used to make the plug body, whether the tip is pointed or rounded, and whether the plug body is straight or right-angle.
The important variations are the diamater of the plug "pin" (i.e., 2.5mm or 3.5mm.) and the number of conductors.
By conductors, we mean the number of individual wires inside the cable attached to the plug, which corresponds to the number of metal contacts on the plug (the tip and the rings). The more conductors and contacts, the more functions the headset can have.
It's also critical that the body of the plug is not too thick to fit into a recessed jack in some phones.
(A) The 2.5mm plug is used for most monaural (one ear) headsets.
(B) The single 3.5mm three-conductor plug can be used for either monaural headsets (with microphone) or stereo headphones (no microphone)
(C) The single 3.5mm plug with four conductors is used for headsets that provide both stereo sound for listening and a microphone for speaking. They are commonly used with smartphones such as the iPod and Samsung Android phones.
(D) The dual 3.5mm plug arrangement is used for headsets designed to connect to PCs. Some PC headsets use USB plugs.
(Photos from Panasonic, MCM Electronics, Plantronics)