AT commands

Suppose you must validate some modem through AT commands. You could follow some steps: checking if antenna is functional, gather IMSI and IMEI number, get network registration status and controls, get operator name and dial a valid number and so on…

AT+CFUN?
AT+CPIN?
AT+CIMI
AT+CGSN
AT+CREG?
AT+COPS?
ATD+<NUMBER>;

You can enter any program that access serial interface (minicom, picocom, etc) to type this commands, but if you like to automate your steps:

Reference: List of AT Commands