Have a look to see if this helps Guide: Text Message Tips (not sent or received)
Good morning @Chris4c , did the above guide give you the information you needed? Need any additional help? 
Cheers @MI5 ! 
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@Marjo wrote:Good morning @Chris4c , did the above guide give you the information you needed? Need any additional help?
Cheers @MI5 !
Unfortunately I've not seen my friend since I posted the question, so I've not had chance to try. Presumably though, the fact that he can send text messages to some of his contacts must mean that the message centre number is correct? It's only messages to some contacts that give the error. All contacts are in the UK.
We'd need more info if that doesn't help.
Has you friend changed from an iPhone?
Are the people they're texting changed phones?
Have they blocked the number etc ?
When you speak to them, ask them to check their network status O2 Service Status
The error could be caused by a network failure or congestion so may be intermittent.
