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.