Safety

Don't attach anything you see in this project to a real telephone network. It's not against the law but they'll get mad, and stuff could break on both ends of the phone line. The Phone Friend emulates the telephone's connection to the Central Office, it is not designed to connect to any telephony networks. At all. 

Additionally, this project generates voltages which I'd describe as "spicy but probably non-lethal." An idle phone line has a standing voltage of -48vdc. When a call comes in, a ring voltage of 90v AC ( yikes! ) is superimposed on top of that. Here is what Wikipedia has to say:

Voltages greater than 50 V applied across dry unbroken human skin can cause heart fibrillation if they produce electric currents in body tissues that happen to pass through the chest area. If the same skin becomes wet, then even voltage sources below 40 V can be lethal.

Since this project ultimately sources it's power from USB, it's very unlikely to injure you. However, it is more than enough voltage to damage any other circuits you might be working near. So even if safety isn't your number one priority, try to keep it in your top three. Three to five. 

All these cool projects have helpful information about interfacing with phones. Thanks for the inspiration!

Bluetooth Rotary Phone - Nathan Seidle, 2005

Interface a Rotary Phone Dial - guidomax, 2009

Simple Phone Intercom - Jason Poel Smith, 2012

Arduino Rotary Phone - Andrew Stella, 2014

Cellular conversion of vintage rotary phone - Johan Berglund, 2016

Rotary Dialer to USB Keyboard - Max Romagnoli, 2017

A Strange Phone - Guy Dupont, 2022

Touch Tone Dial-A-Song - John Park, 2022

Intercom System from Rotary Telephones  - Thomas, 2023