Microtransmitter telephones

From Wiki 24

Microtransmitters, also known as 'comm' units, have both talking and listening capabilities like a phone.

Jack Bauer used a Microtransmitter Telephone to stay in touch with Ira Gaines during Day 1. He has also used them numerous times since to keep in contact with CTU and other field agents, although, until Day 5, in-ear transmitters were only used for covert missions, while transmitters similar to headphones were used on normal missions. In Day 3, the devices appear to be headphones, but during Day 4, the standard issue transmitters have a hook attached to the earbud that fit around the ear. In Day 5, Jack Bauer found a wireless comm unit in a terrorist van, and used it until he gets a new one from CTU before he begins the takedown of Jacob Rossler. He gets a new transmitter with a better range before he goes to meet with the terrorists undercover as Rossler. In Day 6 he and Mike Doyle continue to use it, although Curtis Manning stays with the old ear-hook transmitter.