In the United States, interconnection of mobile radio transmitters and receivers (transceivers) with the PSTN began in 1946, with the introduction of mobile telephone service (MTS) by AT&T. The MTS system employed frequencies in either the 35-megahertz band or the 150-megahertz band. A mobile user who wished to place a call from a radiotelephone had to search manually for an unused channel before placing the call. The user then spoke with a mobile operator, who actually dialed the call over the PSTN. The radio connection was simplexi.e., only one party could speak at a time, the call direction being controlled by a push-to-talk switch in the mobile handset.
In 1964 AT&T introduced a second generation of mobile telephony, known as improved mobile telephone service (IMTS). This provided full-duplex operation, automatic dialing, and automatic channel searching. Initially 11 channels were provided in the 152158-megahertz band, but in 1969 an additional 12 channels were added in the 454459