Phone updates stopped working, and re-registering failed! (uk o2)

On signing up to twitter, I registered my UK o2 pay as you go mobile number, it worked right away, and I was able to update my profile by text message. But after a day, it stopped working all the time, then stopped all together. I tried deleting my device and re-registering, but now it won't register at all. My phone says the message with the code in has sent fine, and I'm using the same number saved in my phone book that worked fine before. But it still wont register! Very frustrating! Any ideas?