Help desire s message not receiving.

[gazza25]

hi my sis in law is having troubles with her htc desire s , she can receive messages but cant send them she has been on the phone to o2 who has said it is something to do with her phone and to take it to an o2 shop to mend , any ideas what can be wrong with it and is there anything we can try to sort it without it costing her , thanx