When I was a child, I was curious about everything. One of these questions, was how a telephone worked. Later on, I learned the related knowledge and started another journey - how to make phone by myself.
Well, if you have the same experience, you may be interested to check this basic but interesting tutorial: make a dialer with Arduino (little programming). Not a whole telephone itself, I'm afraid. I'm making this dialer so when making calls with the telephone (not cell phone or nowadays smart phone), I can just pick up the phone receiver and place the speaker near its mic, type in the number on my PC on Serial Monitor of Arduino IDE, and the number will be dialed accordingly. You may think "now who still uses such old telephone". Well, then just for fun!