Unable to send SMS on 4G with VoLTE enabled

I cannot send SMS messages any more with Chatty. This used to work fine in the past.

I see this in the logging when sending an SMS message:

May 10 20:24:47 avalon mmsdtng[840]: ../plugins/modemmanager.c:cb_dbus_signal_sms_added() Got new SMS
May 10 20:24:47 avalon mmsdtng[840]: ../plugins/modemmanager.c:mmsd_check_pdu_type() This is not an SMS being received, do not care

Because I had to send an important message, I put the SIM-card back in my Android phone, where sending worked fine. What could be wrong? How can I get more details on what goes wrong? Receiving SMS messages still works fine.

Another interesting observation is that there appears a little red cross after approximately 25 seconds next to the SMS messages. So, somehow Chatty knows that something went wrong. But I do not know how to get any additional information, like an error message. In the logging is nothing else than the lines above.

Now I’m thinking of it, the issue of not being able to send SMS messages might have started after a modem firmware update that I did a while ago. But it might also be totally unrelated, it is just a hypothesis.

See also this forum post.

Chatty version: 0.7.2-1pureos1