As multiple users have reported VoLTE seems broken, needs some attention and fix asap. It shouldnt take long to fix I'd like to think.
Same here no VoLTE until I restart my phone. even if I use VoWIFI it will be turned on for sometime then it will stop working again need to restart system.
While I would love to say it's something easy to fix and patch... it's not, but we have people working on it at this moment.
We are aware of the issue and will update you all when I get more information from the R&D team.