This attribute indicates the diagnostic code of the most recently
cleared transport connection.
This indicates the diagnostic code (associated with the
lastTransportConnectionClearCause) provided by the network.
A string containing only a 0 or 00 (zero) indicates that the network
has not provided a diagnostic code for the most recently cleared
transport connection.
Section 5.4.5.15 of the ATMF UNI 3.1 specification lists the
diagnostic codes that can be provided by the network.