If you’ve ever called a business and been told you’re being transferred, they were were likely doing so using a private branch exchange, or PBX. As you might suspect, professional versions can cost thousands of dollars to set up, but as covered in this MagPi article, a Raspberry Pi can do the job for around $100 — including the phones themselves!
The system outlined there features two VoIP handsets — or “endpoints” as they can technically handle more than one extension — Ethernet cables, and a switch. It also has a Raspberry Pi running a RasPBX image to actually route everything, and you could certainly add more endpoints if required.
Configuration is fairly simple, though part of the reason for the “low-low price!” is that the handsets can be purchased on the used market for around $15. This necessitates restoring the phones to factory settings, along with a bit of software setup to get your new phone system running.
Once you’re satisfied that you can talk back and forth internally, the system can be tied to the outside world via a VoIP phone service, and can even be set up to route calls to the mobile phone network. Though you’ll have to pay for these external services, this Pi project looks like a very cost-effective solution for all your landline needs!