Unable to send text messages off network (O2)

Have you tried contacting O2 Support, as nobody here can do anything about a network issue..

As the phone works fine sending to other networks, it seems obvious to me that it's an O2 issue (most likely something they haven't updated correctly after the number change). I don't see how it can be anything else.

You said that SMSs actually leave the phone as normal, but they're just not getting delivered at the other end, so after the SMS has left your phone it becomes O2's responsibility to deliver it. If the phone couldn't send it out to the network, you'd see a red exclamation mark next to it.

I'd give them another call and see if you can speak to someone else, perhaps higher up the Support chain.

I have been having the same problem - i try and text t-mobile (i'm with o2) and according to my phone and o2 delivery receipts the message is received however the other person is not receiving the message. I restored my phone a week ago and that seemed to fix it for a few days but the problem has come back again today!