Hey @SimonB76 ,
I have a case the other day where a third-party app was affecting this feature. Can you please run the phone in Safe Mode to see if this is also your case?
To do this press and hold the Power button until you see Power Off, Restart and Emergency. Press and hold the Power Off icon until it shows a Shield, tap the Shield and the device will restart in Safe Mode.
Safe Mode disables all third party apps and so if this starts working in this mode, we can confirm it is being blocked by a third party app.
Well, it worth a try. Once that the phone software has been upgraded cannot be downgraded, I'm afraid @SimonB76 . Have you checked if the setting is available whilst the phone is running in Safe mode? Are you in an area with good network coverage when the phone network jumps from 4G to 2G or 3G? Have you checked with your operator if this option is enabled on your phone?
I also have missed VOLTE settings on my Samsung Note 10+ Canadian version. Due to the size of the issue, or it is a bug in firmware or VOLTE become to be activated by default and managed automatically (I saw some links mentioning this).
Definitely Samsung should stop playing sick and hide and E-X-P-L-A-I-N.