ring-android/app/src/main/res/drawable/hangup_button.xml

<?xml version="1.0" encoding="utf-8"?>
<selector
    xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:state_pressed="true"
          android:state_enabled="true" >
        <shape>
            <gradient
                android:startColor="#ff8888"
                android:endColor="#cc0000"
                android:angle="270" />
            <stroke
                android:width="3dp"
                android:color="#ff8888" />
            <corners
                android:radius="3dp" />
            <padding
                android:left="10dp"
                android:top="20dp"
                android:right="10dp"
                android:bottom="20dp" />
        </shape>
    </item>

    <item android:state_enabled="false" >
        <shape>
            <gradient
                android:startColor="#cccccc"
                android:endColor="#cc0000"
                android:angle="270" />
            <stroke
                android:width="3dp"
                android:color="#cc0000" />
            <corners
                android:radius="3dp" />
            <padding
                android:left="10dp"
                android:top="20dp"
                android:right="10dp"
                android:bottom="20dp" />
        </shape>
    </item>

    <item>        
        <shape>
            <gradient
                android:startColor="#ff4444"
                android:endColor="#cc0000"
                android:angle="270" />
            <stroke
                android:width="3dp"
                android:color="#cc0000" />
            <corners
                android:radius="3dp" />
            <padding
                android:left="10dp"
                android:top="20dp"
                android:right="10dp"
                android:bottom="20dp" />
        </shape>
    </item>
</selector>