I am not a T-Mobile customer, but a good friend is and he lent me his DATA SIM from his 2012 Nexus 7 while I traveled to save money on the DATA.

I was able to make GOOGLE Voice calls over the throttled DATA without much issue. However, I was using CODECS which are designed for slower DATA connections and have actually made calls in the past over EDGE.

Having a solid 3/3.5/4G connection throttled to 128Kbps made for a much better quality call and a far better user experience in general.

Dan