With 2G/3G being disabled in the UK, phones need to use VoLTE or VoWIFi to call. While Pixel phones from 4 to 7 support VoLTE it needs to be provisioned by the provider/Google. One of the UK providers Lebara cannot do that, pointing fingers at Google, while Google points fingers at the mobile operator. The result is that the users of Google Pixel phones lose the ability of using their phone.

I'm not sure why this isn't provisioned over an OTA update (like it is done with internet settings) but anyway, the fault seems to lie on google here

Yes, I think it is Google's responsibility as well. But it seems one of those situations, where user support is not Google's priority. I know that for Pixel 5 and later there is patch that you can apply (but it only works till the next reboot).

Can some please explain why provisioning is needed and what it does?

There is this article https://www.intraway.com/blog/provisioning-volte

But it seems that basically VoLTE is not enabled by default and each side needs to white list the other. The provider needs to white list the phone and the phone needs some info about the provider. (But I don't fully understand it)

Thanks, that article helps too. As you say, it's a lot of security and identification.