The IMTS, short for Improved Mobile Telephone Service is one of the radiotelephone systems that belong to the “0G” or Zero Generation pre-cellular systems. The system is just an enhancement of the existing MTS or Mobile Telephone System. IMTS was released in 1964. IMTS was the first full duplex mobile system. It was introduced as a replacement to Mobile Telephone Service or MTS and improved on most MTS systems by offering direct-dial rather than connections through an operator.

The initial system had 11 channels operating via VHF high , they are …

Channel , frequency, mobile frequency

5 years later in 1969 12 more channels were added. These added channels operate via UHF. These channels were …

Channel , frequency, mobile frequency

Note: IMTS never operated via VHF low (35-44MHz/ZO, ZF, ZH, ZA, ZY, ZC, ZB, ZW, ZL). These frequencies were only used by the older MTS system.

Any provider might have offered service on one, two, or all three bands, although IMTS was never offered on low band (only MTS). These were prone to network congestion and interference since a radio closer to the terminal would sometimes take over the channel because of its stronger signal. Cellular networks remedied this problem by decreasing the area covered by one tower (a "cell") and increasing the number of cells. Although it was possible to have more subscribers, more towers were required to cover a given area. Because of this, IMTS and MTS systems still exist in some remote areas, as it may be the only feasible way to cover a large sparsely-populated area. As of 2008, at least one U.S. company near the Canadian border was known to offer IMTS service. In locations IMTS and MTS are discontinued, the frequencies used are often allocated to paging systems.

How did it work?

The basic operation of IMTS was very advanced for its time, considering that integrated circuits were not commonly available. An IMTS base station channel was shown to be idle by the transmission of a steady 2000 Hz idle tone. Mobiles would scan the available frequencies and lock on to the channel transmitting the idle tone. When a call was placed to a mobile, the idle tone would change to 1800 Hz channel in use tone, and the 7 digit mobile number would be sent out as pulses, switching between 2000 and 1800 Hz to represent digits. Any mobile recognizing that the call was for someone else would resume scanning for mark idle tone, while the called mobile would then transmit 2150 Hz “received” tone back to the base station. This would also start ringing the mobile phone and when the mobile customer picked up the phone, a 1633 Hz connect tone would be sent back to the base station to signify a connection and the voice would be sent through. When the mobile hung up, a burst of alternating 1336 Hz disconnect and 1800 Hz in use tones would be sent to allow the base station to service another call.

IMTS systems typically had 25 watts of transmitter power at the mobile station and 100-250 Watts at the terminal. In comparison, modern cell phones have a power output of about 0.6 watts. Mobile installations looked and functioned much like a landline, or hardwired, telephone. They were connected directly to the cars batter for power. This made it possible and common that an IMTS telephone would drain the cars battery if used when the car was not running, or overnight. Optionally these units were also connected to the car's horn and were switched to honk the horn as a ringer when receiving an incoming call if the user was away from the car at the time of the call.

The IMTS units were full duplex, meaning that a user could both talk and hear the other party at the same time. This was an improvement over the earlier MTS systems, most of which were half duplex, allowing only one party to transmit at a time; the user had to "push to talk" to speak and then "unkey" the transmitter to hear the other party on the line.

IMTS base station sites generally covered an area 40-60 miles in diameter. This extended range was due to both their large transmitter power and in many cases higher antenna placement at 200–500 ft. IMTS base stations in larger cites had as many as 7 or 8 channels while rural stations had as few as one or two channels. Each telephone connection required the exclusive use of a channel. Because of this limitation these systems had a much lower capacity than do the cellular systems and having all channels busy was common. In larger cities this dictated a very limited number of simultaneous calls.

IMTS technology heavily limited the total number of subscribers. In the 1970s and the early 1980s, before the introduction of cellular phones, there were "waiting lists" of up to 3 years for those wishing to have mobile telephone service. Those on the waiting list were literally waiting for other subscribers to disconnect their service in order to obtain a mobile telephone number and mobile phone service.

These restrictions resulted in very few sales and production of IMTS phones and the mobile units were therefore very expensive ($2000 to $4000). Prior to the division up of AT&T in 1982, Bell System IMTS subscribers usually leased the equipment at a monthly rate of up to $120. Availability of the channels was scarce hence airtime was also quite expensive at roughly $1.00 per minute. Following the division, customer owned equipment was required by Bell companies and monthly rates then typically ran $25 plus air time. Also since there were so few channels it was common for the phones to line up to use a channel and IMTS manufactures competed for the speed with which the units would seize an available channel.

Increasing affordability of satellite service, and government investment in cellular expansion is slowly allowing MTS and IMTS to be removed even in rural and hard to reach area such as the Yukon.