To make the button to answer calls, you would have to use the Accept method of the "Call" class from Zoiper WEB API documentation. You can obtain a "Call" object instance from the OnZoiperCallIncoming callback function.
A very simple example is sited below. Keep in mind that you will need to handle more than one incoming call at a given time, except in the cases where you are sure that there will be no more than one incoming call.
if (IncomingCall != null)
IncomingCall = null;
IncomingCall = call;
Here is also a simple example on how to create the button on your webpage.
<a href= # onclick= namefunction() > Click Accept Call</a>