Change label value according to the dynamic radio button values:

<script language="javascript" type="text/javascript">
        function maskvalidation() {
            var rdolst_0 = document.getElementById("rdolst_0");
            var rdolst_1 = document.getElementById("rdolst_1");
            if (rdolst_0.checked == true && rdolst_1.checked == false) {
                document.getElementById("lblMask").value = "(xxx-xx-1234)";
                document.getElementById("lblMask").innerHTML = "(xxx-xx-1234)";
            }
            else {
                document.getElementById("lblMask").value = "(xxx-xxx-123)";
                document.getElementById("lblMask").innerHTML = "(xxx-xxx-123)";
            }
        }
</script>