Given that it's a house phone, have you tried enabling Wi-Fi Calling (VoWiFi) in the carrier settings (if you have that option), and then putting the phone in Airplane Mode with wi-fi enabled? AFAIK that should be fairly less impactful on battery.

(Alternately, if you don't have the option to use VoWiFi, you could take literally any phone/tablet/etc; install a softphone app on it; get a cheap number from a VoIP number and connect to it; and leave the app running, the wi-fi on, and the cellular radio off. At that point, the device doesn't even need a[n e]SIM card!)