AT+CEREG?
Read command returns the current value of <mode>, the registration
status <stat>, and the information <tac>, <ci>, <AcT> according to the
current <mode> parameter value.
+CEREG: <mode>,<stat>[,[<tac>],[<ci>],[<AcT>]]
<tac>, <ci> and <AcT> are reported only if <mode>=2 and the
mobile is registered on some network cell.