When you bind a callback function to the conversationIncoming event, you can use properties on the conversation object that gets passed into the callback to get the calling number.  There are two properties: callingPartyDirectoryNumber and callingPartyNumber.  Here is an example:
$('#phone').bind('conversationIncoming.cwic', function(evt, conversation, container) {
    console.log('Incoming call from: ' + conversation.callingPartyDirectoryNumber);
    conv = conversation;
});