Default answering call with volume/camera buttons?

As the title says i bought a rock excel series a port of the front of the case is covered with toucwscreen plastic but i can only see the caller id and numer
In accesibility we have the option to end calls with on/off button
So now i only need to get able to anwer a call with the volume or canera key theres an app called ez button but when using it i only get a black screen and mo caller id
Please help me