Linux
https://www.raspberrypi.org/magpi/share-internet-to-the-raspberry-pi-zero/
https://www.raspberrypi.org/forums/viewtopic.php?t=132542
https://dogemicrosystems.ca/wiki/Dial-up_pool#Setting_up_the_Dial-in_Server
Windows
Server side:
https://www.techrepublic.com/article/get-it-done-configure-windows-xp-pro-as-a-remote-access-server/
Client side:
https://ig8g.com/out/utilities/PPP/Establishing%20a%20PPP%20connection%20under%20Windows%207.pdf
The Pi 2 is running FreePBX, with 2 Yealink handsets.
