Advanced Mobile Phone Service, Inc. was a subsidiary of AT&T prior to the Bell System Divestiture.
AMPS was developed by Bell Labs to replace older, severely limited radiophone services, such as IMTS.
AMPS was one of the first modern cellular phone systems, which remained in operation until 2008.
After divestiture, AMPS was divided among the 7 newly created Regional Holding Companies, now known as RBOCs: Through a series of mergers, acquisitions, and re-consolidation among the Baby Bells, these 7 original mobile operating companies are now split as such - PacTel Mobile Access and U S WEST NewVector are owned by Verizon Wireless due to its co-ownership by Vodafone.
Ameritech's Chicago n/w was divested and sold to GTE Wireless in 1999 as a condition of the former's merger with SBC Communications.