In the 20th century the phone was regarded as a “natural monopoly” for several good reasons and perhaps a few bad ones as well.
It was decided that these monopolies would be regulated by “public utility commissions” whose charter would be to set prices to reflect what non-monopoly pricing might be.
Of course that is a very fuzzy charter and PUCs were typically politically motivated.
They were generally careful, however, not to put the utility out of business, as someone might notice.
One trick that they discovered was to require the phone companies (and other utilities too, I suspect) to write off the cost of their plants over about 40 years.
This would result in lower prices to the consumer by pretending that a piece of hardware would remain useful for 40 years.
The utilities had no clear incentive to fight the PUCs on this, for if it turned out a dumb thing to do they could blame the PUCs and indeed by the middle of the century AT&T was delivering phones to the home that were very sturdy and might indeed last for 40 years.
I suspect that AT&T is still depreciating hardware that has been unused for 20 years.
As far as the bottom line goes it is sort of like still paying off a loan on a car that you bought in 1965, and junked in 1975.
To be fair, few foresaw the extremely rapid changes in communications technology.
See my comments on The Fall of the Bell System.