nam

call.nam is a string property containing global path to the call object.

When calling call object functions from for example a setTimeout call you need to provide a unique name of the call object to be called as a string. call.nam contains such a unique name string.

"wrtc.x.call[index]"

When answering a call it is important to call answerCall() of the correct call object. Usually an "Answer" button is displayed by wrtc.onIncomingCall() function and when user klicks on the button answerCall() is called. To be able to call answerCall() of the correct call object you need to use the call.nam property:

wrtc.onIncomingCall = function(call) {
    ...
    status.innerHTML = "<input type=\"button\" value=\"Answer\" onclick=\"" + call.nam + ".answerCall()\" />";
    ...
}

You cannot use call.answerCall() as call is not a global variable.