My iPhone 4S isn't receiving calls?

So I recently upgraded my phone to Apple's new update, iOS 7 and ever since then, whenever someone tries to call me, it goes straight to voicemail, and then appears on my phone as a 'missed call'.It's only on the second or third try that someone can get through to me.
Do you know how I can solve this problem?

Any help greatly appreciated!

If it isn't receiving calls which lets be honest is the basic and fundemetal function of phones then either phone the orange company or whoever you are with and ask for advice, go and get it repaired or replaced immidiatly even if afterwards you have an Abundence of calls to reply to, it will be worth it. Hope I helped