I just installed FreePBX, a linux based open source phone system at work (12 SIP trunks, 40 extensions), and thought, why not a home / small office unit based on a Raspberry Pi? The Raspberry Pi can easily handle multiple SIP trunks, and enough IP phones for the whole family / small business to have a extension or even a private DID. The FreePBX software is free, the Raspberry Pi is inexpensive (about $80 for a complete kit) and PolyCom IP Phones are readily available for around $80 each. Now you can have individual voicemail (email integrated), auto attendant, intercom, paging / bells, and music on hold just like the big boys. The phones and paging adapter can be powered by a inexpensive POE network switch, as can the Raspberry Pi with a POE Hat.
Arduino integration? Yes your Arduino can make outbound calls with prerecorded alerts!
Become the Maker you were born to be. Try Arduino Academy for FREE!