I recently debranded my Orange San Francisco 2 (ZTE Blade 2) and now it won't send sms messages.

I can receive sms messages, and I was able to send messages a couple of hours ago (after it was newly debranded) so how can I fix this? Usually I can sort out my problems with a quick google or a reboot, but this doesn't seem to work for this.

 

I should add that every time I write a message and press 'send', I get a notification straight away telling me that the message hasn't sent.