I sent in a 70$ trade in iphone 8 in december. T-mobile said they were giving me a free Iphone 13

T-mobile is saying that they don’t see the $70 trade in phone in the system.  So, as a result, are now CHARGING ME OVER $700 for a “Free iphone 13!”  THEY lost my $70 old Iphone and now they *3 months later) started billing me monthly installments for a FREE PHONE!  I said, ok - I’m a reasonable guy, my time is worth more than the 45 min of holding for 2 mins while the agent asks the supervisor...I said, bill me the $70 for the phone that YOU GUYS LOST!  I will write off the fact that my time is worth more than this, but to all the sudden charge me months later and simply set up a payment plan for 700+ dollars is absolutely NOT OK!  The process is, they fill out a form and they “look for the phone”  why wouldn’t you look for a missing phone before starting to bill me?   Now I am waiting a week to see if they find it.  She said if they don’t find the 70$ phone, they are billing me over $700?!!!  I’m not a mathematician - but how does that math even work?