OnStar relies heavily on North American AMPS service for its subscribers because, when the system was developed, AMPS offered the most comprehensive wireless coverage in the US. ADT recently asked the FCC to extend the AMPS deadline due to many of their alarm systems still using analog technology to communicate with the control centers.[7] Cellular companies who own an A or B license (such as Verizon and Alltel) must still provide analog service until February 18, 2008. After that point, however, most cellular companies will be eager to shut down AMPS and use the remaining channels for digital services. OnStar is transitioning to digital service with the help of data transport technology developed by Airbiquity, but warns customers who cannot be upgraded to digital that their service will permanently expire on January 1, 2008.[8]
http://en.wikipedia.org/wiki/AMPS