Google Voice still routes calls over your mobile minutes (your phone calls google's servers, which forwards the call to the number you're calling, so your minutes are still taken up while talking).

This can be circumvented with apps like GrooveIP which will route the call over data first, contacting Google's servers and then completing the call like normal.

This is especially useful with a decent wifi network as GrooveIP "uses 1.2MB per minute" per their app description.

EDIT: for the math impaired, this amounts to a little over 4300 minutes not counting any other data usage for the $30 100 minutes, unlimited texting, 5GB 4G data plan on T-mo. When your data is throttled to 56kb/s, your connection may not be strong enough to maintain a phone call, as 56kb/s is equivalent to roughly .42MB/min