Transfer existing T-Mobile number to another person

hi everyone, im a new member and i've been search for help on existing t-mobile #. recently i have 2 #'s in my t-mobile family plan that i dont need anymore. i called t-mobile to cancel those 2 #'s but they said i will have to pay a total of $400 for an early cancellation fees. however they said there is service or website online that is those who want to transfer or looking for existing #. i think the method is that the person will take over my existing # and its good for both side. tat person will not have to pay for new contract fee and i dont have to pay for my cancellation. and also after the # is being transfer they can change the # to a new #. so the only thing is for them to agree to carry on the # but doesnt have to keep the same #. im sorry if i confuse anybody. but if there is anyone who know where i can get help please let me. thank you!