VoLTE provider support

Without reading the whole thread I want to add that VoLTE on O2 Germany and X10II registered and worked at the beginning but is not being able to register anymore. I have no clue when it stopped working, but rebooting and re-inserting the SIM didn’t help.
Any ideas?

edit: thigg solved it with a hard reset of the device and this also solved it for me. VoLTE works again.