Default CoDec on GTalk

I know what CoDecs are supported by GTalk; however, I would like to know what is the default CoDec used on GTalk. So, can anyone please tell me what is the default CoDec used by GTalk? Can it be changed? The reason I ask these questions is because I have my asterisk (runs on LaFOnera WiFi router) configured to use iLBC as the default CoDec for GTalk; however, the connections seemed to use a PCMU CoDec. PCMA/U CoDec is too expensive for some of my friends who only have about 1GB/month bandwidth allocation. So, I would certainly like to use any CoDec that consumes the least bandwidth that can be freely supported by asterisk running on a Linux embedded device supported by any open-source firmware communities.