E.118 specifies decimal digits, but many real SIMs carry hex characters
in EF_ICCID (e.g. China Mobile's 898600B5... range, some MVNO/IoT SIMs).
AT+QCCID returns them verbatim, and the strict isdigit() check blanked
the ICCID, leaving the modem daemon stuck in INITIALIZING forever and
cellular dead. ModemManager parses ICCID as hex for the same reason.
Verified on a comma four with a China Mobile SIM (EG916Q-GL): previously
stuck retrying 'identity read incomplete', now dials and passes traffic.
Daniel Koepping