RADIUS attribute from rfc2865

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Callback-Number

This Attribute indicates a dialing string to be used for callback.
      It MAY be used in Access-Accept packets.  It MAY be used in an
      Access-Request packet as a hint to the server that a Callback
      service is desired, but the server is not required to honor the
      hint.

   A summary of the Callback-Number 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

      19 for Callback-Number.

   Length

      >= 3

   String

      The String field is one or more octets.  The actual format of the
      information is site or application specific, and 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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