Could it be a network carrier setting somewhere?
How is it that you can't send SMS? The composer is just the text box right... or is it that SMS arrive as MMS?
EDIT:
Disclaimer - I've not tried any of these codes myself, so no idea if they will help you!
If you're receiving a sending error, you could try the following codes to set the T-mobile SMS centre number (+447958879879):
**5005*7672*+447958879879#
I presume the +44 prefix is fine, otherwise replace it with a zero. Also some users report just using a single * at the beginning.
Seems you can also view the current setting with *#5005*7672#
Tried those codes but nothing happens...just gives me a success message but when i go into my message and try to compose an sms text it still comes up as a mms message which then sends as mms.