I have same problem, both my work (o2) and personal (o2) SIMs work fine in another handset, neither work on wifi/VOLTE in my Samsung. Have reset the samsung 3 times, now cleared catche, new SIM, attempted to raise faults with Samsung, raised faults with O2 - some users have reported to have resolved to go the issue resolved but I am one of the unlucky one. Wifi/VOLTE works for exactly 24hrs then stops - you can time it to the second. Hopefully there is a silver wand out there.
@Elly62 Hi, not sure how, but I think it looks like have got my wifi/VOLTE calling working now and its settled (touch wood) I have done a number of things so not 100% sure what I did to be honest that cured it. More recently I backed up - wiped and re-installed everything barr the setting.
Have you tried to wipe the cache partition? Also when you get wifi calling, try placing wifi as the preferred network (not mobile network), also when you lose the wifi-calling option option are you actually connected to the wifi at the time? Also try turning off Samsung RCS setttings in the message settings.
What network SIM did you try in the handset? Also did the handset come from O2 or another network?
One option is to log as a fault and ask O2 to re-provision your line however it sounds like a handset issue I faced.
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