When the phone begins to ring, can you accept or reject the call with a voice command?
LG Nexus 5 running 6.01
You can try the app Voice Call Answer SMS Reader which claims to do what you are wanting :
Now answer incoming calls and read SMS content by giving voice command to your phone. First of its kind app. Simply say YES or NO to pick or reject the call or to read SMS content. This Call Answer and SMS reader app can be very useful if you are driving.