# How can I attach a microphone and speaker to the Arduino GSM shield?

I recently purchased the Arduino GSM shield so that I could build my own low-tech cellphone. Now that I've successfully been able to send and receive text messages, I would like to add the ability to make voice calls.

I was able to find a schematic in the modem documentation that describes how to build the earphone circuit. It seems fairly straightforward but I have a few questions:

1. What does "differential layout" mean?
2. What is the purpose of the transient-voltage-suppression diode in this circuit?
3. What type of device is the "amphenol 9001-8905-050"? Based on the diagram it looks like 3.5mm 4 pole phone jack.