Vomitorius
What comes into my mind when I read this: What says IMS status in phone information (##4636##)? Is it registered or not?
First you can try resetting mobile network settings...
If that doesn't help dive into the APN settings of your SIM card and check the options in sub-menu 'APN type' with marked entries 'default', 'mms', 'supl'...by scrolling down a bit you'll find the option 'ims' and mark it (if it's not), save and exit APN settings. Switching flight mode on and off afterwards or a device reboot should now show the options for VoLTE/VoWiFi in SIM settings.
I experienced the same with two german carriers months ago where I couldn't place and recieve calls.