
Smartphones such as the iPhone, Samsung Android phones, Blackberrys and Palms have 3.5mm jacks, normally used to play stereo music through "headphones." You can also use a wired telephone "headset," but will need one equipped with a 3.5mm plug, or a 3.5mm-to-2.5mm adapter to use with a headset that has a 2.5mm plug. You can also use a wireless Bluetooth headset.
Don't confuse headsets with single 3.5mm plugs (for phone use) with headsets with dual 3.5mm plugs (for computer use).
(phone jack photos from www.myownposting.com and www.pocketnow.com)