This object specifies the backward signal the gateway sends to report national network congestion, covering congestion on national trunks, congestion at terminal exchanges, or an abnormal timeout/release of the R2 register itself. Because this single signal covers three distinct congestion scenarios per the R2 spec, seeing it fire frequently is a strong operational hint to check trunk-group utilization and register-timer settings together rather than assuming a single root cause. If a terminating exchange's R2 register times out waiting for a digit that never arrives, it may issue this congestion signal, which the originating gateway should log and treat as a call-blocked event rather than a subscriber-busy condition.