Change Button Image on Button click

Selector.xml

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://ift.tt/nIICcg">

    <item android:drawable="@drawable/btn_call_active_big" android:state_pressed="true"/>
    <!-- pressed -->

    <item android:drawable="@drawable/btn_call_normal_big"/>

</selector>

And on button background

 <ImageButton
            android:id="@+id/btnPhone"
            android:layout_width="0dp"
            android:layout_height="40dp"
            android:layout_weight="1"
            android:background="@drawable/button_background"
            android:src="@drawable/btn_call_normal_big" />

Please advice on above . where i am doing wrong