HTC Desire - Won't receive texts?

I've had my HTC Desire for over a month now, but from the start, it won't receive texts if I use the internet at the same time, or if someone else texts me too. Phoned 02 to complain but said I needed a password which my mum forgot. It now doesn't receive texts when I use any apps, text more than one person, on the internet, or sometimes even when I'm not using it. It can't be to do with the signal, because I've always got signal.. Help?
 
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