iOS 15 VoLTE issue

[Lfcdanny]

Recently upgraded to an iPhone 13 Pro from a XS. I no longer have the ability to chose which mobile network I wish to connect to, where I used to be able to disconnect from 4G and connect to 3G as it was stronger in my area. 
I've recently become a postman after leaving O2 as an employee and found I am unable to also turn off VoLTE/4G Calling. I cover an area of the northwest where VoLTE/4G calling is very ropey and calls very frequently become robotic and drop as a result of not having this option available. I’m not sure this is the right place for me to post this but can O2 update their carrier settings to allow users to turn this setting on/off as we were able to in previous iOS versions? 

[MI5]

@Lfcdanny 

Unfortunately it's called progress........

[madasaf1sh]

@Lfcdanny 

I can still do this on IOS 15.3, so nothing to do with carrier settings, but it does look to have also have affected AT&T and some other networks globally, so looks like an OS restrictions. 

What is your carrier settings version?

Ignore the Honest Mobile, I am running in Dual Sim mode on my iPhone SE with Dev OS's

edited to add question

[Lfcdanny]

Hi, thanks for the reply. 
I can see that you have the option available, as i believe it didn’t seem to be a problem using non 5G devices like yours, the iPhone SE. My partner has a 5G device in the iPhone 12 and also cannot change this setting.

 My carrier version is O2 48.0