That would be consistent with the symptoms. Big telco networks are hierarchical with most functions pushed to regional data centers with a very small number of services in a redundant pair or trio of central data centers. Subscriber database (HSS, UDR) would be one such function.
The cause of a failure of the HSS could be manifold, ranging from router failures to software bugs to cyber attack (databases of 100M+ users being a juicy target).
One slightly scary observation from NANOG was that FirstNet, the network that ATT built for first responders, was down. That would be ugly if true and I'd expect the FCC to be very interested in getting to the bottom of it.
The cause of a failure of the HSS could be manifold, ranging from router failures to software bugs to cyber attack (databases of 100M+ users being a juicy target).
One slightly scary observation from NANOG was that FirstNet, the network that ATT built for first responders, was down. That would be ugly if true and I'd expect the FCC to be very interested in getting to the bottom of it.