Hello, I tried all the examples of the MKRGSM library and I have the same problem each time.
The modem does not respond correctly to AT + CPIN?
I receive ERROR systematically after AT+CPIN?
My SIM card does not have a PIN, so I used #define SECRET_PINNUMBER "".
I also tried gsmAccess.begin(NULL).
The SIM card works very well in a SIM900.
I have a 2000 mAh lipo battery full loaded.
The blink example works correctly (LED on pin 6 blink).
The TestModem example works correctly, i receive the modem IMEI.
The GsmWebClient stops at gsmAccess.begin.
In debug mode (GSM gsmAccess(true)

i can read :
AT
OK
AT+IPR=921600
OK
AT+UPSV=3
OK
AT+CPIN?
ERROR
AT+CPIN?
ERROR
AT+CPIN?
ERROR...........