You are charged once the message reaches the O2 server.
From there it is sent to the receiving network and if it fails for any reason at this stage there is no way of the system knowing.
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)