[Zope] ZCatalog Question - Seach on numbers, not letters

A set of DTML Documents have a field containing telephone numbers. The users want to be able to search, for example, for all telephone numbers starting with a given area code (like 713). The field has been indexed using a TextIndex, a FieldIndex, and a KeywordIndex. In each case, the index was set up and the index process was run. For the TextIndex case, no objects were (re)indexed and the subsequent search results were always empty. For the case the KeywordIndex and the FieldIndex, only exact matches were returned. Is there a way to index something like a telephone number (i.e., no letters) so that they return valid results on -partial- input? Thanks in advance, Ron