How much does it cost to change my cell phone number on t-mobile?

People I don't know keep on texting me and calling me and I want a new number so no one knows it. They won't stop even if I tell them off they pretend I never said anything.How much does it cost to change my cell phone number on t-mobile?
It costs $15 to change your cell phone number usually. But if you call customer care from your phone and explain to them what is going on they may waive it for you.