Hi pix,
T200 which to Kassem refers supervises Ack on the LapDm (layer 2 protocol on the Um interface (MS-BTS)).
In this case BSC and core network are unaware of that and need to be advised by Error Indication message (from BTS to BSC) in such a way to release the call.
In case of T200 expiry on Abis interface, BSC is already informed and no ERRIND is necessary.