If your carrier supports VoLTE, there is a way to fix this issue, go to your phone settings, then go to about phone, right here you will see your phone number, is it showing as Unknown or showing the number?
If it shows unknown, IMS registration status will show not registered. Lets fix this the same way i did for myself, take out your sim card from the phone, now you need another android phone to do this. don't use samsung devices. This case I used my old Lenovo VIBE SHOT, insert the sim card to another phone then go for Sim Management and put your sim card number manually with following country code like this +966XXXXXX
Then turn on mobile data for 2/5 minutes if you see 4G then plug out the sim card and insert it into your Samsung device now you will VoLTE.
