Why can I receive a text message but not send one?

I do not know your phone, but try changing the Message Center Number, probably listed under the Messages menu-Message Settings-Text Messages-Sending Profile-Default Profile, or your phone settings menu.
Write down the old Message Center Number in a notebook before you overwrite this, just in case you need to use it again if the new number does not work.
Message Centre Number in my phone is
+447785013998
This works for a Vodafone within the UK.
Or try a similar number from a friend's phone on the same network as your own, for example O2 Genie, Orange, T-mobile.
Let me know if this works.
Jim.
jim_mckee_@msn.com

Jim McKee
October 2005