T-Mobile: http://www.t-mobile.com/cell-phone-plans/mobile-internet.htm...
U.S. Cellular: http://m.uscellular.com/uscellular/plans/data-only-plan.html
http://www.popsugar.com/tech/Data-Only-Plans-26912644
Also, T-mobile offers free international data in many, many countries. This may offer a cheaper option than international voice calls for callers with traditional plans.
EDIT:
Also, and this may be T-Mobile wide, I am experiencing that they are routing all voice traffic over the data line, which means poor coverage now results in ch-op-py dr-op-ou-t speech just like a bad SIP line would. I tried to make a call last weekend and the other end sounded just like a modem it was so bad.
Just caveat emptor.