AT commands
Updated
AT commands are the text commands used to control a cellular modem through a serial interface, such as AT+CGDCONT to set the APN or AT+CEREG? to read the network registration. The commands for 3GPP modems are specified in 3GPP TS 27.007.
In practice
When a device does not connect, three commands answer most questions: AT+CPIN? (is the SIM ready), AT+COPS? (which network is selected) and AT+CEREG? (is the device registered, searching or refused).