The AT command set originated with dialup modems (specifically the Hayes smartmodem), and has been extended for use with cell phone connections.
This command set is interpreted by the modem (or cell phone). You will find lists of these commands
, though they can vary slightly between devices.
For example, ATDT*99#
is a AT
tention command to D
ial using T
one dialling, and by convention, *99#
is the access point.