When I get an incoming call, on my home screen or lock screen, no option to answer pops up. The phone rings, I even click on the phone button and it pulls up my recent calls while still ringing! I can make calls fine. And I was able to do so at first. Then one day I couldn't.
Hmm.. Odd. Go to Settings > Apps > All > look for the Phone app, make sure it is not disabled then tap clear cache and clear data.