Your T239 lacks a wifi radio, so it's not possible. On the G1 it would be possible on the other hand. Here's one method which may still work if you don't use your G1 actively.
http://economyofeffort.com/2010/07/30/turn-an-android-handset-into-a-free-wifi-phone-with-google-voice/
Definitely look at the Howardforums link in the thread Bob suggested above.
Here I sit with 2 T-mobile phones.
#1 is my Samsung T239
#2 is my G1

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