GSM Shield Hardware for voice communication

Hello,

I'm currently trying to setup voice calling to talk and hear from the GSM shield. I've been following the GSM shield documentation: http://arduino.cc/en/Guide/ArduinoGSMShield#toc11, but i cannot get any clear voice. I do not believe my problem is with the software as i can establish the call. Once the call is establish i can speak into a Electret microphone (http://www.mouser.com/ProductDetail/PUI-Audio/AOM-4544P-2-R/?qs=sGAEpiMZZMtcsMZaWNSquzKrJpH74dgY%2FkbvN%2FGaWEw%3D) and hear it from my phone. The problem is that the voice is hidden in much louder static sounding noise. The same happens when i call the GSM shield and listen through a speaker (http://www.mouser.com/ProductDetail/PUI-Audio/AS02808MR-R/?qs=sGAEpiMZZMuTkJYgZlQcSQjjFmMST0FVPLlFjfmpem0%3D) i purchased.

One of my problems may be the component that looks like the back-to-back zener diode in a circle in the schematic. I'm not quite sure what this is, can anyone tell me? If it is some kind of diode can anyone specify what properties it should have? Also i do not have anything for the "Differential layout". Must i have something here? According to the pictures in the tutorial, it doesn't look like they include and differential layout or the previous component i'm uncertain about. Thank you.