Make sure that both ends of the phone cord are plugged in and secure (they will click into the phone socket when connected)
Ensure the phone cord isn’t cracked, worn or damaged
Make sure if you are using cordless phones that the phone base has power and phones are properly being charged.
Try another phone jack in the house
Try unplugging all devices from the phone jacks except for one. This ensures no other equipment in the house is causing a problem. Try each phone separately to isolate a faulty device.