Attribute Number: 19
  Value: String

  The value of this attribute is the number to which the RADIUS client
  gear should return a call to the authenticating user. Depending on
  what packet this attribute is found in, different actions may be
  configured. For instance, if Callback-Number is found in an
  Access Request packet, the implementation may assume that the
  end user is requesting callback service. If the attribute is found
  in the Access Accept packet, it can mean everything that the
  administrator configuring the gear wants it to mean. In fact, in
  some cases, Callback-ID and Callback-Number will not be found
  together in one packet.