These days, most land-line telephones have extra bells and whistles that require them to be plugged into power as well as the phone outlet. Some are remote cordless units that transmit and receive with a base unit. The original, plain old telephone service (POTS) requires no extra power. These simple phones get all the needed power from the 12 volts coming from the central office. If you have a POTS landline, be sure you have a plain old telephone to go with it! A plain old telephone is obvious because you only plug it into the phone jack and not also into the electrical outlet. If the power goes out, it still works. My preference is to buy an older but working AT&T or Western Electric on eBay because they were built to last. The local Big Lots or Target should have newer versions for less than $20.
During the first hour or two of an widespread emergency it will seem line the telephone service is 'out.' Simple explanation : central offices are sized to the population, using formulas for how many people will be on the phone at an average time. In an emergency, everyone picks up the phone at the same time and vastly exceeds the 'formula'. You don't hear dial tone because the switch is too full to provide it instantly. If you stay on the line, you will get Dial tone, possibly in a few minutes as others hang up. Unless you need 911 service immediately, hang up. Try again in an hour or two. By then the real emergency calls and general 'DID YOU FEEL THAT!?!' calls will have subsided and you should get dial tone fairly quickly.
Yes, I worked for the phone company at one time in my life... I also lived this phenomenon after the 1989 Loma Prieta earthquake in the sf bay area. In a wide-spread emergency, be patient and HAVE A PLAN (that you have practiced) so you don't need to rely on cell phones or landlines to have some certainty about where your family members are.