Seeed Studio GPRS Shield v2.0 Audio Jack

I've been working with the Seeed Studio GPRS Shield in a project for a flame effect controller. The shield and underlying SIM900 module are great; sending and receiving calls and SMS messages has been very easy.

A problem with the shield that I've run into is that there is very little documentation for the two-in-one audio jack. It's a 3.5mm four-connector TRRS (tip-ring-ring-sleeve) jack for the handset speaker and microphone:

The shield schematic shows the following for the audio jack:

Testing the continuity of the connection to components on the board confirms that the pinout for the jack should be:

Tip Speaker +
Ring 1 Speaker -
Ring 2 Mic -
Sleeve Mic +

This is obvious in retrospect: visualize it as the audio connector is inserted upwards into the bottom of the schematic symbol. The pins connecting the jack to the board don't seem to be numbered sequentially counterclockwise from the upper right; the S+ pin (4 on the schematic) is on the lower left, and the upper left pin is not used.

The standard 4p4c wiring for a phone handset has four wires: black, red, green, yellow. Black and yellow are for the speaker, red and green are for the microphone. To use a standard phone handset with the shield, wire the TRRS connector this way:

TRRS 4p4c
Tip Yellow
Ring 1 Black
Ring 2 Red
Sleeve Green

The dial-a-flame-effect details post will undoubtedly happen at some point soon.