We can find out if the SIM is locked by issuing:
$ sudo mmcli -m 0
...
-------------------------
Status | lock: 'sim-pin'
| unlock retries: 'sim-pin (3), sim-puk (10)'
| state: 'locked'
| power state: 'on'
| access tech: 'unknown'
| signal quality: '0' (cached)
-------------------------
...
SIM | path: '/org/freedesktop/ModemManager1/SIM/0'
...
To enter the PIN for SIM 0:
$ sudo mmcli -i 0 --pin=<pin_number>
successfully sent PIN code to the SIM
This changes the state as expected:
$ sudo mmcli -m 0
...
Status | lock: 'none'
| unlock retries: 'sim-pin (3), sim-puk (10)'
| state: 'disabled'
...
In case we introduce the wrong PIN three times, we need to enter the PUK to unlock the SIM card:
$ sudo mmcli -i 0 --puk=<puk_code>
successfully sent PUK code to the SIM