A way to determine if a certain character contains a phone number

<script language= "javascript"
<!--
function Checknum (str) {return!/\d{6}/.test (str)}
function Check ()
{
if (document.form.tell.value== "") {
  alert ("Please enter the phone number!") ");
  Document.form.tell.focus ();
  return false;
}
else{
  var letters = "0123456789 () +-";
  for (i=0; i< document.form.tell.value.length; i++) {
     var Checkchar = Document.form.tell.value.charAt (i);
     if (letters.indexof (checkchar) = = 1) {
       alert (" The phone number is not in the correct format! ");
       Document.form.tell.focus ();
       return false;
     }
   }
 }
}
//-->
</script>

<form method= "POST" Name=form action= "" >
<p align= "center" >
Please enter the phone number: <input type= "text" name= "Tell" size= ">"
<input type= "Submit" value= "submitted" name= "B1" ></p>
</form>