My calls are going straight to voicemail when screen locked on my iPhone 4s

I bought the iPhone 4S two days ago and because it's the weekend I didn't notice an issue with my calls and texts until now.

When my home screen is locked, the text message alert tone doesn't sound. Also when someone tries to call me it rings once then goes straight to voicemail. I followed all the steps for fixing problems with iPhone on the Apple site (resetting network settings etc) and it didn't work.

Any help you guys can give would be much appreciated because it probably goes without saying that's it's crucial I'm able to recieve calls.

Many thanks!