T-Mobile SIM card in iPhone?

I currently have a prepaid T-Mobile plan for $15/month. If I buy an unlocked iPhone, will I be able to insert the SIM card into the iPhone and keep the prepaid plan that I'm currently using but with the iPhone?

Yes You can keep your plan, any sim pay as you go or contract will work in any unlocked phone including iphone's.
i was told just the other day in my friends phone shop. Enjoy!

Yes… But you may have a data plan attached with that. Keep that in mind.