[Zope] Zcatalog's TextIndex Question

Can anyone explain why TextIndex only works with words but not numbers? For example, I have a catalogaware ZClass with a property PONumber of type String. I created a PONumber as TextIndex in the Catalog. I created 3 instances of the ZClass with PONumber Verbal, Tim Dunken, 289-4569. When I search for Verbal, Tim, or Dunken, I get the result. But searching for 289, 4569, or 289-4569 returns a blank page. How can I solve the problem? Regards, Ping