This read-only field reports why an attempted trunk backup failed, distinguishing between buTrkNotDef, no backup trunk has actually been defined or configured for this primary, and buTrkNotEstab, a backup trunk is defined but the switch could not establish the connection. An admin uses this to immediately tell apart a configuration gap from an operational failure, which require very different fixes, one needs provisioning, the other needs troubleshooting the actual backup call path. For example, lportBuFailReason reporting buTrkNotDef(1) tells the admin no backup trunk was even configured for this primary, so the fix is provisioning one, whereas buTrkNotEstab(2) means a backup trunk exists but something, like the far-end call setup, is actually failing.