JavaScript Phone Format

function fnPhoneNo()
{
if (event.keyCode < 48 || event.keyCode > 57 ) event.returnValue = false;
if(document.getElementById("txtPhoneNo").value.replace(/^\s+|\s+$/g,"").length >= 3)
{
if(document.getElementById("txtPhoneNo").value.replace(/^\s+|\s+$/g,"").length == 3)
document.getElementById("txtPhoneNo").value += "-";
if(document.getElementById("txtPhoneNo").value.replace(/^\s+|\s+$/g,"").length == 7)
document.getElementById("txtPhoneNo").value += "-";
}
}