Trade in credit issue

Traded in iPhone back in November of 2025 and never received trade in credit of $600 that was promised have called customer service multiple times and always promised that it was being fixed or someone will follow back up and it never happens. The phone now has less the 600 due to be paid off and I’m owed more than that in credits. Also been told multiple different answers as to what happened with the traded in some say it was never received and some agents say that it was received and I will receive credit shortly but each time I call they have no record of previous conversation for where everything was left off. I probably have 10+ hours in phone calls to resolve at this point and I’m just over trying to get everything resolved.