Wedescribe an industrial-strength data mining application in telecommunications.Theapplication requires building a short (7 byte) profile for all telephonenumbersseen on a large telecomnetwork. By large, we meanvery large: we mainrain approximately350 million profiles. In addition, the procedurefor updating these profiles is basedon processing approximately275million call records per day. Wediscuss the motivation for massive tracking and fully describe the definition and the computation of one of the more interesting bytesin the profile.