I suggest you load up the SerialGSMPassthrough sketch from the examples and enter "AT" which should be followed by the prompt returning with OK. If that works then try the "AT+CIND?"
which will return a series of values to indicate your battery status, sim card status etc etc.
AT+CIND=?
+CIND: ("battchg",(0-5)),("signal",(0-5)),("service",(0-1)),("sounder",(0-1)),("message",(0-1)),("call",(0-1)),("roam",(0-1)),("smsfull",(0-1)),("gprs",(0-2)),("callsetup",(0-3)),("callheld",(0-1)),("simind",(0-2))
OK
AT+CIND?
+CIND: 5,3,1,0,0,0,0,0,2,0,0,1
OK
Further information available in the u-blox AT command manual.
https://www.u-blox.com/sites/default/files/u-blox-CEL_ATCommands_(UBX-13002752).pdf
you could also make your own sketch and send this command rather than using the pass through option, whatever works for you.