Unable to send SMS on US Mobile GSM (T-Mobile MVNO)

[NoahAndrews]

I'm able to receive SMS texts, and I can both send and receive phone calls, but I haven't been able to get SMS sending to work. I'm using the correct +1123-4567 number format. I've tried both chatty and modem-manager-gui, but the issue seems to lie somewhere between modem-manager and the actual modem.

Steps to reproduce and debug:
1. Send SMS via chatty
2. Run mmcli -m 0 --messaging-list-sms

    /org/freedesktop/ModemManager1/SMS/5 (unknown)

3. Run mmcli -m 0 -s 5

-----------------------------
  General    |      dbus path: /org/freedesktop/ModemManager1/SMS/5
  -----------------------------
  Content    |          number: +1XXXXXXXXXX
            |            text: test
  -----------------------------
  Properties |        pdu type: submit
            | delivery report: requested

4. Explicitly tell mmcli to send the message, by running mmcli -v -m 0 -s 5 --send

[03 Jul 2020, 13:39:45] [Debug] Assuming '5' is the SMS index
[03 Jul 2020, 13:39:45] [Debug] ModemManager process found at ':1.9'
[03 Jul 2020, 13:39:45] [Debug] SMS found at '/org/freedesktop/ModemManager1/SMS/5'

error: couldn't send the SMS: 'Timeout was reached'

Any help on this would be appreciated.