We will first have to type “AT” A response "OK" should be returned from the mobile phone or GSM modem. Now we will type "AT+CPIN?". The AT command "AT+CPIN?" is used to query whether the mobile phone or GSM modem is waiting for a PIN (personal identification number, i.e. password). If the response is "+CPIN: READY", it means the SIM card is ready for use. After this, various AT commands depending on the used modem’s instruction set are send to modem and responses are received. This process is very useful in testing GSM modems.