Charged after number was ported to another company.

I was a Tmobile customer in 2018.  I ported my number to another company in June, then in September I was contacted by Tmobile collections that I owed for services from June to September.  Why was I charged for months after my number was ported?  My account should have been cancelled after my number moved from Tmobile.  I've been trying to get clarification, but they keep giving me a false collections number to call.  I call the number and all I get is a debt resolution law firm that doesn't know anything about Tmobile.  What do I have to do to get some answers?  Then I get told to go into a store to check the details of my account.  Why can't customer care do this?  I've gone into the store before about my billing, and they just tell me to call the customer care line.  What do I do?????  UGH!