Quantity   Component name
1 × 7 Segment LED
1 × Teensy 3.1
1 × 10uF capacitor blocks the DC between the phone and ADC input
1 × 5V isolated DC-DC switcher + capacitors Adds 5V to 5V. The telephone won't work with less than about 8V bias
1 × 2.2K & 10K resistors For DC bias at the ADC input pin
7 × 150 ohm resistors
1 × Old 1980s era telephone.....
1 × 2K 1/2W resistor (not in photo) between the power and phione
1 × Battery pack + hack to keep it from turning off