At the opposite end of the scale
There's BT (OpenReach) and their approach to faultfinding on the local loop for DSL faults.
An ADSL modem is a marvellous piece of technology, and the people that did the early standards did a grand job.
The hardware (the DSP) has effectively already got a time domain reflectometer for locating discontinuities in the cable.
The low level DSL protocol has "last gasp" messages so the owner of the exchange kit can tell the difference between a controlled shutdown and an unplanned loss of signal e.g. due to a fault.
The low level DSL protocol also exchanges details of signal quality, error counts, useful stuff like that, and the DSL kit at each end makes the numbers available to the user (your SNR, CRC, FEC, HEC, etc).
All this regardless of vendor.
Posh kit might even have SNMP (e.g. the DSL Line MIB) in addition, but it's not sufficiently common to be widely useful for diagnostics.
All in all, a veritable galaxy of faultfinding assistance just waiting to be used, generally accessible equally well from the consumer kit or from the exchange kit (if anybody's sufficiently motivated).
What's the typical approach to DSL faultfinding at OpenReach?
Ignore all the above, wait for an unhappy customer to phone in, ignore all the above, threaten to charge £100+ for a faultfinding visit, ignore the builtin diagnostics and hope that if a technician does show up the fault is visible at the appropriate time, and then rely on adequately clued-up technicians to use their portable testgear to do much of the same diagnostic stuff that is inherently and unavoidably built into the DSL modem.
That manual intervention approach might have worked for an installed base of a few hundred modems per exchange.
I think we got a bit further than that in recent years,
Just like FaceBook, manual intervention doesn't seem appropriate on that scale. Unlike FaceBook, the DSL diagnostic stuff is already built in **and published**. Unlike Facebook, where broken kit may mean lost revenue, BT see "rent-a-technician" as a revenue opportunity.
Well done Facebook.