My and my wife both have Samsung Galaxy S5 phones and have been encountering the same issue. Our phones will just randomly answer an incoming call but will not ring. The caller gets sent straight to voicemail. If the user of the phone see's that the phone call is active and try's to talk to the caller then neither the person holding the phone or the caller will hear anything. I have tried everything I can think of to disable this. I even took my phone to the best buy where it was purchansed and had a guy from Samsung flash the firmware of the phone. That did not fix anything.