Walter,
Your point and concerns are valid. But I suspect the availability of land-line phones during power outage is something of a lost cause these days, thanks to proliferation of mains-powerred DECT phones, which are often the only thing households posses, and few of which will work in an outage. I know it used to be a requirement to have one wired phone, and it may still be, but I'd say it's universally ignored.
The first DECT phone I ever purchased, a BT diverse 1010, came with a backup battery pack containing 6 AAs that fitted in the PSU lead, so it would be useable in a power outage. But that phone cost well over £200 in 1997, a far cry from todays bare-bones £20 supermarket bargains.
That said, I'd add...
d) another concern (of depending upon mobiles) is that in the event of a major incident, even if the base stations are still working, the service soon becomes unuseable due to cell overload as everybody at once reaches for their mobiles. I believe that's been a regular occurrence during the terrorist attacks of the last decade, such as 11/9 and 7/7. A similar 'overload' phenomenon affects land lines too of course, but to a lesser extent owing to the 'five nines' service objectives.