In days of yore, a land line telephone was powered by its own electrical current - supplied by the phone company. The phone company did this to insure telephone service was not terminated if the home's electricity was shut off (for any reason).
Here's my understanding of "modern" land line phone service: A modern land line phone is, in most cases, a VoIP phone (VoIP = Voice over Internet Protocol - i.e. the land line piggybacks your internet service). The land line only operates when there is an internet connection. The internet connection is provided by your home computer. Your home computer is powered by your home's electrical power. Bottom line, phone service available only for so long as there is electrical power to provide internet connectivity to your home computer. So, if your home's electricity is shut off, you lose that VoIP phone service.
Long gone are the days when the loss of electrical power to you home did NOT cause you to lose your land line phone service. [Of course, you can use your cellphone, but only if you can recharge your cell phone through other power sources - your car's electrical connection, etc. But that is different from your land line phone service.]