<stat>: network registration state
For <stat> = 1 or 5 (i.e the Wireless CPU® is registered on a
network) the next parameter couple is returned if <mode> = 2
For <stat> = 3 (i.e. the network registration is denied) and if the
customer application has enabled, the denied network
registration cause with the +WBHV command, only the
<rejectCause> parameter is returned with the meaning as
explained in <rejectCause> mode
0 not registered, ME is not currently searching for a new
operator
1 registered, home network
2 not registered, ME currently searching for a new operator
3 registration denied
4 unknown
5 registered, roaming