Other platforms probably detect the method automatically.
O2 Germany doesn’t support SMS via IMS, for instance it is not possible to send SMS through a VoWifi-only connection (e.g. with an Android device).
I don’t expect much to change (as well as I don’t expect VoWifi to be implemented in the foreseeable future) due to low ressources @jolla. The workaround (temporarily disabling VoLTE to send messages) works fine, so at least for me it’s not much of a problem since I barely use SMS anyways.
It also seems necessary to disable VoLTE on O2 Germany to call the Congstar virtual mobile network, because otherwise the connection would be terminated instantly as soon as the called party answers the call. Some minor issues, well, it’s beta status, after all.