Can’t send sms with ModemManager “GDBus…: Unknown error”

I have an internal mobile modem well detected and currently working for 4G internet. BTW, ModemManager also detect it:

% mmcli -L
    /org/freedesktop/ModemManager1/Modem/0 [Hewlett-Packard] MU736

So, when I try to send, I start by creating an sms:

% mmcli -m 0 --messaging-create-sms="text=Test,number=002127…"
Successfully created new SMS: /org/freedesktop/ModemManager1/SMS/3

The SMS is correctly set:

% mmcli -s 3                                        -----------------------------
  General    |       dbus path: /org/freedesktop/ModemManager1/SMS/3
  -----------------------------
  Content    |          number: 002127…
             |            text: Test
  -----------------------------
  Properties |        pdu type: submit
             | delivery report: not requested

But, when I try to send it, it failed:

% mmcli -s 3 --send
[06 mars 2022, 07:50:31] [Debug] Assuming '3' is the SMS index
[06 mars 2022, 07:50:31] [Debug] ModemManager process found at ':1.15'
[06 mars 2022, 07:50:31] [Debug] SMS found at '/org/freedesktop/ModemManager1/SMS/3'

error: couldn't send the SMS: 'GDBus.Error:org.freedesktop.ModemManager1.Error.MobileEquipment.Unknown: Unknown error'

### Edit Sometimes I also can get this kind of error message:

% mmcli -s 4 --send
error: couldn't send the SMS: 'GDBus.Error:org.freedesktop.ModemManager1.Error.Core.Connected: No AT port available to run command'

So, what does it mean and how could I solve it?