RADIUS attribute from rfc2865
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.