The message centre in my n95 8gb on the 3 network is the number "3"
My mate had exactly the same problem the other week on o2. they culd receive, but not send texts. They changed the message centre number and it worked straight away. Their message centre number was a normal +44 **** ****** number.
You can find the message centre number following the steps below mate. It should start +44 and you can get it from service provider. To change it, just highlight it and choose options then change.
messaging
options
settings
text message
message centres
sim message centres
message centre number