This seems to be an ongoing problem with lumia 720. We have used iOS and android and wanted to try windows trusting Nokia and MS. What a waste. Such a beautiful phone, We will have to sell it. We can't tell the problems we have faced with this phone. Once I had to make an urgent call and this phone went into sleep. We lost the faith that the phone will be available to do the most basic requirement of a phone , I.e. Making a call.
Sorry Nokia & MS, we are very much disappointed.
A disappoint loyal Nokia user