I have the same issue (with my 10iii) not being able to send SMS when VoLTE is enabled. I’m on the network of the largest provider in Denmark, Yousee.
In addition I have to other issues which I think is related to the way VoLTE is implemented.
-
I receive the same SMS several times, or an download error, due to a faulty receipt acknowledgment from the phone to the sms-server. This issue is regardless of VoLTE being on or off. But the problem did not exist before VoLTE.
-
Some (maybe 50%) of incoming calls fail with an error message the there is no network connection, but the network connection is there, I can call back immediately with out problems. My feeling is that even if VoLTE is off the phone does not drop the connection to 2G for some calls.
All three issues disappear when I move the same sim-card to my XA2 - which by the way work much better after upgrading to 4.6.0.13.
I have tried everything including total reflash first of Android 11 and then SFOS 4.6.0.13, nothing helps, so I have to use my old XA2 and for second year running keep the 10iii in the drawer. I have pre-ordered the C2 phone as I doubt my 10iii will ever work with SFOS.