If you're a T-Moblie customer and spend most of your time calling other mobile phones, it looks like the carrier is planning to launch a new feature that might be worth investigating. According to a document obtained by TMoNews, T-Mobile will start offering unlimited mobile-to-mobile minutes (regardless of carrier) as a $10 monthly add-on to existing voice plans. The leaked documentation notes that you can add this feature on and not worry about having to sign a new contract, and it doesn't affect the rate plan that customers already have — it's just an add-on feature. T-Mobile's not the first carrier to think of this — all of Sprint's plans already come with unlimited mobile-to-mobile minutes, and AT&T offers it as an add-on to go along with its $20 unlimited text messaging feature. If you're interested, this feature is reportedly rolling out on April 4th... and Verizon will be the only carrier without an unlimited mobile-to-mobile option.