I'm new to O2. Since I changed, my wife can no longer text me without getting a text fail message. It says the text will be received when the user (me) is back online. If she selects the error there is the option to resend by sms/mms. Which works...we have tried deleting text app cache, deleting all messages and removing each others contacts. All to no avail. The problem persists. It doesn't effect any of her other contacts, just me. And I do not have any problems texting her...?
Check the advanced messaging option found in message settings, you may have got "chat" toggled on and you will need to turn the setting off.
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.