A cell phone is a two way radio. In technical terms, they use what is called a "duplex" communication channel, which allows communication in both directions simultaneously. So, both persons can speak into the phone at the same time.

A radio that only allows one speaker at a time uses a "simplex" channel.

That being said, I think the Talkabouts use a simplex channel, since only one person can speak at a time (I think).

Thanks everyone for your replies.