InCallUI/res/drawable/ring.xml

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

    <stroke
        android:width="1dp"
        android:color="@android:color/white"/>

    <size
        android:width="52dp"
        android:height="52dp"/>
</shape>