Unfortunately that isn't possible.
O2 do not record or store phone conversations.
The only way to do it is for you to instal an app on your phone that gives you the ability to record if it happens again in the future.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)