Billing Incorrectly

i had an upgrade in January 2018, for a Samsung S8, for some reason i am still billed for my old phone and this was never explained to me, why would you say i am due knowing you still going to bill me for something i am not using and its difficult to get through the right department. i need this to be resolve as soon as possible. i have been upgrading for years but this is the first that i have to pay for a new phone and the old phone.