This read-only field reports why an incoming ISDN call attempt on this port was rejected, per its description limited to two named causes: an authentication failure, or the available B-channel or resource pool being fully busy. An admin uses this to immediately distinguish a security or credentials problem, worth investigating as a potential intrusion attempt or misconfigured CPE, from a simple capacity issue, meaning legitimate calls are being turned away because all channels are in use. For example, if lportISDNCallRejCause consistently reports the pool-busy cause during business hours, that is a strong signal to provision more ISDN B-channel capacity rather than to suspect a security problem.