!!! Charles Manson !!!
I had a car phone in my back in the late 70's. each time you were getting ready to release to listen, you had to say "Over" (similar to using a Ham radio). !
There were different types of mobile phone systems over the years, some essentially had a two-way radio system with an added "phone patch" otherwise known as a "Marti." I do not remember what "Marti" stands for, Motorola called them a "radiotelephone interconnect"
TRUE old school mobile phones of the time period you speak were "duplexed." Most two way, CB and amateur stuff is "simplex" meaning that first one guy talks (transmits), unkeys, then the other party keys up and talks.
"Duplex" or "full duplex" in this context means that both parties ---just like a landline phone---can talk and hear the opposite end at the same time. True mobilphone units had a "duplexer" installed in or alongside the radio and in fact did just that---transmitted and received in duplex.
Lots of pictures of equipment and history of early mobile phone stuff from here:
http://www.wb6nvh.com/Carphone.htm
As a side note, I used to run amateur radio gear from my Roadrunners while stationed at Miramar from 70-74. San Diego area amateurs STILL HOLD the same call for one area amateur repeater, WBWLV. Back in those days it's main in/out frequencies were on "two meters" but they had a UHF control link up on 450. I had an old G.E. "Pre Prog" (A name conjured after the fact---radio was made before G.E. Progress Line came out, so AFTER PL came out, the early line was dubbed "pre prog." Anyhow, I had a 450 receiver and cavity filter in the trunk, and could hear the "talk back" on the repeater as I was transmitting. We thought we were so damn cool back then.
A few of us also Located San Diego's "mad jammer" but that is another story.