RADIUS attribute from rfc2865

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Called-Station-Id

This Attribute allows the NAS to send in the Access-Request packet
      the phone number that the user called, using Dialed Number
      Identification (DNIS) or similar technology.  Note that this may
      be different from the phone number the call comes in on.  It is
      only used in Access-Request packets.

   A summary of the Called-Station-Id Attribute format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |  String ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-Type

      30 for Called-Station-Id.

   Length

      >= 3

   String

      The String field is one or more octets, containing the phone
      number that the user's call came in on.

      The actual format of the information is site or application
      specific.  UTF-8 encoded 10646  characters are recommended, but
      a robust implementation SHOULD support the field as
      undistinguished octets.

      The codification of the range of allowed usage of this field is
      outside the scope of this specification.

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