function check_fields(){
	if (document.Form.pre.value == "" || !isNumeric(document.Form.pre.value) || document.Form.pre.value.length < 3 || document.Form.pre.value.length > 3 )
	{
		alert("You must enter a 3 digit phone number prefix.");
		return false;
	}
	else if(document.Form.suf.value == "" || !isNumeric(document.Form.suf.value) || document.Form.suf.value.length < 4 || document.Form.suf.value.length > 4 )
	{
		alert("You must enter a 4 digit phone number suffix.");
		return false;
	}
	
}
function isNumeric(str)
// returns true if str is numeric
// that is it contains only the digits 0-9
// returns false otherwise
// returns false if empty
{
  var len= str.length;
  if (len==0)
    return false;
  //else
  var p=0;
  var ok= true;
  var ch= "";
  while (ok && p<len)
  {
    ch= str.charAt(p);
    if ('0'<=ch && ch<='9')
      p++;
    else
      ok= false;
  }
  return ok;
}