It used to be all basic telephones were powered by the central office which had diesel generators to recharge the batteries in case of commercial failure. While the likelihood of a power failure is fairly low, nasty storms or power company failures do create local outages that can last for 24-48 hours. During such an outage telephone service is especially critical since roads may be blocked. PBXs had batteries and hand generators for ringing.
It seems most people at home have cordless phones powered by house current. How long, if at all, will such phones last in a power failure? People are supposed to have a plain landline phone, but do they?
But I wonder today how many business PBXs have any battery backup at all for both talk and ringing, especially for an extended period of time.
Some organizations have their own diesel generator power backup, but fail to test the switchover on a regular basis. This is vital because without good testing a changeover will fail when it is needed. (trust me on this)
I know the cable TV system in my area depends on commercial power and stays out even after power is restored (I guess they have to reset it). That scares me if I dependended on cable TV for telephone service.
I presume cell phone towers and relay stations have some battery backup, at least I hope they did. But is it enough?
Likewise with "concentrators" or whatever is used for amplifiers between the home and distant Central Offices if direct copper isn't supplied. I think years ago they were supplied by CO power but I don't know about today.