try taking your sim out and cleaning the little gold chip on it if you are still having problems after then try a new simcard (nip to an o2 store and get a replacement for free) your details will be transferred to it within 24hrs
My answer: I'm afraid the O2 network doesn't support the delivery report function.
However, there is a way you can still receive a delivery report, just follow these two simple steps every time you send a text message:
1. Make sure the delivery report function on your phone is switched off. 2. At the start of your message, type in *0# (don't worry they won't see this) and leave a space before your message. For example: *0# Hello, John. How are you today?
This will usually let you receive a delivery report, but please be aware that we can't guarantee this with every message