You could google for : “gsm based door intercom” to get an idea of what features you would like to implement e.g. unlock main entrance door. You’d also have to think what would happen if the house phone was already in use at the time the visitor called.
You can also look here for a building block for making cell phone applications: Overview | Arduin-o-Phone | Adafruit Learning System
and see some commercially available systems here to get an idea of what features to implement: Wireless Intercom System Review for 2022 by Intercom Experts