This happens with iMessage as well as SMS ?
Strange as iMessage does not require any special settings or provisions. Just make sure iMessage is showing as activated OK and that you have your phone number entered correctly.
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)