You can get the currently used PLMN with the AT+COPS?
command. From 27.007:
+COPS? +COPS: <mode>[,<format>,<oper>[,<AcT>]]
...
Read command returns the current mode, the currently selected operator and the
current Access Technology. If no operator is selected, <format>, <oper> and <AcT>
are omitted.
....
<oper>: string type; <format> indicates if the format is alphanumeric or numeric;
long alphanumeric format can be upto 16 characters long and short format up to 8
characters (refer GSM MoU SE.13 [9]); numeric format is the GSM Location Area
Identification number (refer 3GPP TS 24.008 [8] subclause 10.5.1.3) which
consists of a three BCD digit country code coded as in ITU-T E.212 Annex A
[10], plus a two BCD digit network code, which is administration specific;
returned <oper> shall not be in BCD format, but in IRA characters converted from
BCD; hence the number has structure: (country code digit 3)(country code digit 2)
(country code digit 1) (network code digit 3)(network code digit 2)(network code
digit 1)