I tested your suggestion, without quotes:
AT+CSCS=GSM
Error
If I do it with quotes:
AT+CSCS="GSM"
OK
On the other hand, when entering the commands:
AT+CSCS?
+CSCS: "GSM"
OK
AT+CSCS=?
+CSCS: ("IRA","GSM","UCS2")
OK
Could it be that the right syntax for the USSD (AT+CUSD=1,"*444#",15) is different?
It’s worth to note that prior to the CUSD command, I enter AT+QCFG="nwscanmode",2 in order to bring the module to 3G (in many cases the USSD queries don’t work with LTE networks).
Thanks in advance for your help!