To choose a codec you want in MSN messenger?

Ok. This is a movie codec site, I know, but I found that there is quite much knowledge around here, so I’d like to pop up a question, to which some of you might know the answer:
MSN Messenger 4.7 (and 4.6..., SIP-based applications!) provides quite many audio (and video) codecs. It automatically decides which codec to use, mostly it uses red/8000. I’d like to manually decide which codec to use and compare them. How can it be done? I know it can’t be done in MSN Messenger settings, but is there a tool to manipulate the codecs? MSN Messenger is not mandatory, any VoIP program will suit for me, if the problem is solved by changing the software. Just want to study by myself (not from the books) the codecs in different kind of traffic scenarios.

Thanks, Peter