How much does it cost to change your phone number with T-Mobile? & how do I do that?

I%26#039;ve been have alot of prank calls and texts from weird people I don%26#039;t even know. I%26#039;m kinda freaked about the prank calling. They call and I say who is there and stuff. Then they don%26#039;t answer. But I hear them breathing! So it might be like When A Stranger Calls. Scary! Well any answers will help! Thanks! :D|||call customer care to ask them to change it. it cost 15 bucks that will be applied to ur bill.