Javascript to validate to Phone No in CRM 2011

function checkisinteger()
{
  var s=Xrm.Page.getAttribute('telephone1').getValue();
  if(s!=null)
  {  
    var i;
    for (i = 0; i < s.length; i++)
    {
        // Check that current character is number.
        var c = s.charAt(i);
       if (((c < "0") || (c > "9"))) break;
    }
    if(i    {
     alert("Enter a valid phone number");
     Xrm.Page.getAttribute('telephone1').setValue("");
    }
 }
}