function validateTelephone(input) {
  var regex = / ^(1\s*[-\/\.]?)?(\((\d{3})\)|(\d{3}))\s*([\s-./\\])?([0-9]*)([\s-./\\])?([0-9]*)$/;
  var telephone = input;
  
  if (telephone.match(regex)) {
    return true;
  } else {
    alert("The phone number entered is invalid.");
    return false;
  }
}