Sony Ericsson Xperia X10 Mini/pro :: Keeps Ringing - Change Settings?

It keeps ringing! I had a couple of texts, which I read, then the phone just kept on ringing. Totally bizarre. I tried everything - changing the settings on ring tone etc, vibrate only, nothing has worked. The only thing that works is turning it off & on again (but when I receive another text, off it goes again!). I eventually just turned down the volume but obviously this is not practical. I drove all the way home with it ringing merrily beside me!

I saw on another forum someone else has had this problem, but no solution was offered. I tried changing the ring tone, but even after choosing another tone, it still rang with the other tone, ie 2 different rings at the same time! A friend says her blackberry does the same, and requires the text to be acknowledged. I tried this, even deleted the messages, but no joy.