Things are a bit more complicated than that. First of all there are loop carrier/multiplexer units in the field that have very limited battery backup, some as short as 2 hours, rarely longer than 7 or 8. If you're on one of those, a week's backup at the CO is useless. As for DSL, I'm not sure there is any requirements for the CO equipment to have battery backup, as it is a relatively new and unregulated service. Also, since divestiture the old Bell System rules have generally been phased out to maximize profits. There was strong telecom opposition to the FCC rules I mentioned in my last post.
Unfortunately, I think Bill is right - phone service these days is "when we feel like it". On the other hand, the telcos haven't replaced a lot of the older switches, which had decent backup power. The original purpose of the battery backup wasn't to maintain service in the event of power failures, rather it was to save maintenance costs associated with replacing batteries in phones on customer premises. In fact, the Bell System saved over a million dollars the first year this was installed over 120 years ago, a nice bundle at the time that got better as they converted more offices..
Battery backup is expensive and the batteries have a short life (about
20 years in a CO building, only about 5 years in outside plant). Often the backup generators were installed as the building was being built and as the switches were upgraded the load dropped considerably. Some telcos have agreements with local utilities to drop off the utility, go on generator and feed the excess capacity back to the grid, all for a reduced rate from the utility. Utilities use it to help their supply through peaks. Of course the old stuff does wear out and replacements cost money, if you can even get them in the building....
Wired loops are on the way out, being replaced by fiber and radio. Don't expect any improvements to POTS and recognize that expensive and rarely used things like backup power will degrade.
Just my two cents...
PS - As an interesting aside, the power failure in New York back in
1965 shut the city down for a few days. Calling volume, however, more than doubled as everybody had to call everybody and tell them the power was off. New York Tel had those magic "message units" billing and they made out like bandits. The reserve power group at Bell Labs got a nice boost in their budget for a few years after that profitable experience. These kind of events only happen rarely though, so they aren't often seen in the quarterly profit statements...