[1] On October 2, 1946, Motorola communications equipment carried the first calls on Illinois Bell Telephone Company's new car radiotelephone service in Chicago.
[2][3] Due to the small number of radio frequencies available, the service quickly reached capacity.
The channels are identified by pairs of letters taken from positions on a North American telephone dial that, when changed to digits, form (for 12-channel mobile sets) 55, 57, 95 and 97.
[clarification needed] In the 1960s plan, the VHF high-band allocations provided for 11 channels in the United States: JL, YL, JP, YP, YJ, YK, JS, YS, YR, JK and JR.
The driver for replacement in most of North America, particularly large cities, was congestion, the inability of the network to carry more than two dozen channels in a geographic area.
Currently, the only viable solution is satellite telephony, as the small number of "base stations" which orbit the planet serve large geographic regions as they pass over.
Soon enough, customer equipment, "selective call", was developed that had a built-in circuit that could be programmed to recognize a five-digit code as its own, in a manner similar to IMTS systems and crude compared to cellular phones.
The operator would connect to the base station by cord board and key the five-digit number; in the 1990s, phone operators at TOPS positions would key the five digits after dialing the code to initiate the call and then identify the base station; a typical TOPS operator code would consist of a two digit sequence for voice-call or selective call, a three digit sequence for the base station, then the customer number.
Calls from mobile units to a base station were started by signaling the operator by pressing the microphone button.
These services required far fewer base stations and were used to reach distant locations over vast territories.
Northwestel, which discontinued the service shortly after 2000, could not tie this system into TOPS, and had to aurally monitor the channels using speakers to listen for incoming calls.
For billing purposes, many MTS base stations were identified with a very close-by rate center of an automatic exchange.