Message send failure

[Anna099]

Hello

since 4 May I have received a “message send failure” note for every text I have sent to five of my contacts (my dad, two of my work colleagues and two of my friends). 
It happens every time. 
Some of the messages are getting through, because sometimes I get a reply. But I don’t think they are all getting through. 
Can anyone help me get this problem sorted out?

[pgn · Accepted solution]

Does the message get through if you key in the phone number afresh to the SMS app as you start creating the message, @Anna099?

I had this problem sending to a short code from my address book, had to delete the number from the phone's contacts and re-enter it.

Failing that, check this guide, Guide: Text Message Tips (not sent or received) first, then check each contact can receive a manual SMS (not using your phone's contact list).

If you still have problems, make sure the intended addressee has not inadvertently blocked your number.

If that fails, it's going to be a call to O2, either on the phone (numbers here: Guide: Coronavirus Community Help and Support, or via Social Media (links below), @Anna099. Good luck!

[Anna099]

Thank you for your reply it was much appreciated. 
Unfortunately none of the solutions have worked. I will contact customer services.