Itemised bill for WiFi calls

[#1]

How can I get this and who should provide this? Mobile network provider or WiFi provider?

[#2]

Your mobile provider - calls go through their network, wi-fi merely enables you to make the calls without using 3/4/5G.

[#3]

Turns out the specific issue I’m having is with regard to 0800 (free) calls and O2 say that it is mentioned in the contract that these may not show.

Thanks for your help