I need to add: The modem index and SIM index are not fixed. So the full correct way to get this info is
mmcli -L
to list the modems. For example, gives: /org/freedesktop/ModemManager1/Modem/1
So the modem index to use at this moment in time is ‘1’.
So
mmcli -m 1
You can check, in passing, your own phone number (| grep own:) and get the SIM index (| grep SIM)
The output for the SIM index looks like:
SIM | dbus path: /org/freedesktop/ModemManager1/SIM/1
So the SIM index is 1.
So finally
mmcli -i 1
-i and -m are abbreviated forms of --sim and --modem respectively.