Nokia Lumia 620 - What to do if the call answering keys are not working on my Lumia 620 phone?

Call answering keys may not work properly if your fingers (one or several) are touching the display at the moment when the phone starts to ring. When this happens, the call answer and call end keys are grey and do not function at all and the call can’t be answered or ended. Also the call answer by sliding your finger doesn’t work in this situation.You can fix the problem by pressing the lock key twice.