Randomly unable to receive calls

A few weeks ago I got a HTC Desire on Virgin Mobile (I believe they use T-Mobile's network) in the UK.

I'm having a bit of a problem with it, though. At seemingly random points, if somebody tries to phone me, it will go straight to my voice mail. If I then try to phone them back it will work. And usually, if they try to phone me back after that it will work too.

At first I thought it might be a problem with my phone being on standby, as that's when it seems to happen the most. But I have received calls when it's on standby and I've also had a few calls go straight to voicemail when the phone isn't on standby too.

Does anyone have any ideas why this might be? I've tried looking through the phone settings for anything obvious that could be causing this problem but I can't see anything.