IMTS

IMTS stands for Improved Mobile Telephone System.  This was an automatic telephone system that was in usage right after MTS - a manual car telephone system and before cellular systems were introduced.  IMTS car telephone systems were popular in the US during the 1970s and for the beginning of the 1980s.

IMTS just as its forerunner MTS system operated at lower frequencies than cellular does today.  These systems operated at VHF (about 150 MHz) and at UHF (about 450 MHz) frequencies, and as such, vied for attention with television - which operated at similar frequencies.  Since the frequency space was limited, not many car telephone conversations could be on the air in any given city.  It is perhaps not surprising that television often won the battle for primary usage.  Thus these systems were not well known, even though for a single mobile conversation, they worked well at the time.

When cellular arrived in approximately 1983 in service, IMTS systems of course rapidly fell out of usage.  The cellular systems operating at higher frequencies could offer much more service to many more customers, and thus mobile telephone became popular as it is today.