ERROR: Timeout when calling AT _ | elapsed ms = 0_ ERROR: begin() failed at AT DEBUG: Verbose monitoring and modem serial access enabled DEBUG: Write Modem Serial = AT+CGATT? ERROR: Timeout when calling AT+CGATT? | elapsed ms = 0 DEBUG: Write Modem Serial = AT+CGATT?
If the first AT command fails it means the Uno cannot talk with the modem. Make sure the TX & RX are right. Remember the modem’s TX > Uno RX & modem RX > Uno TX. You’ll need to know the shield’s pinout.
Make sure for data transmission between the sim900 and arduino using pins 7,8 make sure that the jumper of the GSM card is positioned in D7 and D8 to activate these pins.