If I understand you correctly, you have Zoiper WEB on both sides. but you can not hangup the call by using your custom button?
The example in the documentation just a very simple example. It doesn't take into account the incoming call and doesn't store it in the ActiveCall variable.
For example if you change the OnZoiperCallIncoming to this
ActiveCall = call;
The hangup is actually going to be called in this case because ActiveCall will not be NULL.
For an other approach you can for example modify the hangup function like this.
var call = Zoiper.Call;
if (null != call)
Which will always hangup the current call.
Note that this are also examples and you have to modify them or implement something completely different to suit your needs.