usually you can get quite satisfying results by using the GSM codec if you have a throtteled data connection as well. It has a nominal data rate of 36 kb/s including headers. I think the T-Mobile data is limited to 128 kb/s, so that should me more than sufficiant.
Keep in mind that the latency of the connection is a more severe issue when you are registered to 2G networks.
PS: if you like it scientific i recommend the following read:
http://www.sersc.org/journals/IJCA/vol6_no4/30.pdf