If text messaging is enabled on your P+ account page (the one you access via the website), try disabling it, waiting a bit and re-enabling it. If that doesn't work, ask a CS person nicely to do the same thing (if you haven't already gone this route).
The only other thing I could suggest is re-flashing the phone firmware.
For most phones, text messaging is pretty easy to get working, in my experience. I use the same phone and it worked immediately upon activation.