Improved Mobile Telephone Service

When the mobile hung up, a burst of alternating 1336 "disconnect" and 1800 Hz "seize" tones would be sent to allow the base station to service another call.

The mobile would then respond with its identification, consisting of its area code and last four digits of the phone number sent at 20 pulses per second, just as in inward dialing but with the addition of rudimentary parity checking.

Mobile installations normally consisted of a "head unit" or the telephone handset which sat in a cradle with a direct dialing keyboard.

Unlike cellular handsets, these units passed through a dial tone when the receiver was lifted from the cradle and in this way seemed more like a landline telephone.

There was a separate large radio transceiver chassis, typically measuring at least a foot square and 6 inches high, mounted either in the trunk or under the seats of an automobile.

In 1960 General Electric introduced the "Progress Line" DTO- series MTS mobiles which were full duplex, although subscribers were still required to press the "push to talk" bar on the handset to speak.

These were essentially modified "walkie-talkies" with a DTMF (dual tone multi-frequency) keypad attached on the front panel, which fooled the terminal into believing an IMTS mobile was using the system.

Typical IMTS briefcase phones were made by Canyon, GCS, SCM Melabs and Livermore Data Systems.

Two lights on the "head" indicated busy (red) if no channels were idle and in-use (green) if connected to the tower, or depressing the push-to-talk switch.

The VHF and UHF frequencies have been opened to other services unrelated to mobile telephony and largely reassigned.

In the 1970s and the early 1980s, before the introduction of cellular phones, there were "waiting lists" of up to three years for those wishing to have mobile telephone service.

These limitations resulted in low quantity sales and production of IMTS phones and the mobile units were therefore very expensive ($2,000 to $4,000).

Following the divestiture, customer-owned equipment was required by Bell companies and monthly rates then typically ran to $25 plus air time.

Increasing affordability of satellite service, and government investment in cellular expansion allowed MTS and IMTS to be removed.