All the controls you need to answer and end calls are right on your AirPods, so you don't need to touch your phone at all while you're wearing them unless you want to reject an incoming call.
When a call comes in and you are already wearing AirPods, double tap anywhere on the outside of either AirPod. It doesn't matter whether you have first- or second-generation AirPods.
If you don't want to accept the call, do nothing until it goes it voicemail, or reject it in the usual way using your iPhone, such as pressing the side button or rejecting the call using the on-screen controls. You can also double-press the Force Sensor to decline and send it immediately to voicemail.