breach of contract

on 4-19-24 i entered into a contract to upgrade my phones 0.00 end cost for the 2 yr contract, i would be billed and credited each month for the 2 yrs., I never received the credits got billed over 400.00 what the heck do I do I have contacted them countless times and I am always told i will receive a contact back when they figure it out but never get return contact.