iPhone drops calls despite having good signal (iphone 5s iOS 7.0.5)

I first noticed this problem after updating my iPhone 4S to iOS 7. Myself and wife have now upgraded to the iphone 5s but the problem is still there. Specifically, we will be in the middle of a conversation, anything from 2 to 5 minutes, and the call will drop despite having a viable signal strength showing. The loss of call begins with the person you have called being unable to hear you, although you can hear them perfectly. Then after another 5 seconds or so their voice is lost, then after a further 2 to 3 seconds the 'call failed' tone and message appears. I've spoken to a couple of friends with same phone/system combo and they are having the same problem. Can anyone shed any light on this?