Not sure of the terminology, but if TT PSTN infrastructure is similar to BT's (which it will I'm sure) ......... local Exchange switches will be routed to a 'Super-node'. Around my way, half my patch are routed to Manchester, whereas the other half is routed to Preston.
If you'll indulge me, I remember nearly 30yrs ago and a particular System X suite when down at Preston Super-Node, and it was down to a 'Battery Sensing Fuse'.
I can't remember the ins and outs, as I was a Power Construction engineer at the time, but our Power Maintenance brothers mentioned that this tiny milliamp fuse had popped and had caused the entire power-cabinet feeding the suite to shut down !! A million memo's later and all sense fuses were either removed or replaced, can't remember which ??
Something as simple as that caused what was classed as a MSO (Major Service Outage).