show the main window (if hidden) when a new call is received

src/net/java/sip/communicator/impl/gui/main/call/CallManager.java

@@ -482,6 +482,9 @@ public void incomingCallReceived(CallEvent event)
         if (mainFrame.getState() == JFrame.ICONIFIED)
             mainFrame.setState(JFrame.NORMAL);
 
+        if(!mainFrame.isVisible())
+            mainFrame.setVisible(true);
+
         mainFrame.toFront();
 
         this.callButton.setEnabled(true);