All Sizes and Types
Cellphones typically use sub-mini and mini-phone jacks for the headset. For example, this iPhone accepts a TRS music headset for stereo playback as well as a TRRS telephone headset, which uses the fourth line for the microphone and phone control (answer/end call). Phone, mini-phone and sub-mini connectors are used in myriad devices, primarily for audio applications (see phone connector and mini-phone connector).
Is the Line Busy?
Tip and ring dates back to the late 19th century such as when this PBX was switching calls for only lawyers in Richmond, Virginia. In order to find out if an extension was in use, the operator touched the tip of the plug to the outer ring of the extension socket on the board. Known as "tipping," if static was heard, the line was busy. (Image courtesy of AT&T.)
Learn more about tip and ring