Traditional telephone service has historically been more reliable than CATV service due to it's lack of active gear on the poles and remote from the central office.
With traditional phone service each customer has their own wire pair all the way back to the local telco central office switch so there isn't anything but wire on the poles and the central offices have been built to the old Bell Labs standards with substantial backup power.
CATV has always had active gear on the poles in the form of line amplifiers and now fiber optic "nodes", so there has always been an issue with needing to provide backup power at all those remote locations.
The situation is rapidly changing now where growth of urban sprawl and the need to provide data and video services to every customer has made it necessary for the telcos to shift to the remote terminal model which puts active gear in the field just like with the CATV system and subjects them to the same remote backup power needs.
Basically the old notion of the telephones always working is going out the window just as rapidly as the old notion of the cable never working. In many areas where the telcos RT transition is in place, which also correspond to areas where the cable operators are the most up to date, both services are of comparable, reasonably high reliability.