Some people have reported success using the $10 unlimited data plan for non-smartphone along with a $29.99 300 anytime minute voice plan. This come out to the same price as the $39.99 data only plan, but on the plus side, you have another 300 minutes.

This is perhaps a little dishonest as T-Mobile expects you to be using this plan with a very cheap phone that won't/can't consume that much data. Your ethics are your ethics though.