This is a really interesting solution to a relatively common problem of implementing a whole house intercom system. Keep in mind, many of today’s phones already have an intercom feature, but it isn’t nearly as satisfying to use as one based on an Arduino. You can really learn a lot about electronics and micro controllers by reading about this project. Check out the link for the circuit diagram, code, and very detailed description of the entire build process.
The intercom temporarily disconnects premises telephones from the Telco line, and rings them with a distinctive cadence. After a suitable ringing period, pick up your phone and wait for someone to answer. When you take your phone off hook, (or someone else on premises goes off hook,) the intercom stops ringing and supplies premises phones with talk current. All off-hook parties can converse. When all premises phones go back on hook, the intercom re-connects them to the Telco line.
In the Maker Shed: