[HELP] T-Mobile Predatory Bill Collection

After canceling our T-Mobile service, our final bill was sent almost immediately to a debt collection agency called AllianceOne. We also could no longer see our account to see charges because they deleted it online. We emailed AllianceOne requesting verification of the debt but never received a response. In the meantime, T-Mobile continued sending us bills with additional late fees, even though the account had already been sent to collections. Now we've received a new collection notice from a completely different agency, Diversified Adjustment Service, and the balance has increased by about $50 due to accumulated late fees. I called Diversified Adjustment Service to ask what was going on. They said they believe AllianceOne may have returned the debt to T-Mobile, and T-Mobile then assigned it to this new collection agency. So now we're dealing with a second debt collector, additional fees, and still no response to our original request for debt verification. Has anyone experienced something similar with T-Mobile? Can they keep adding late fees while the debt is in collections, especially when we requested verification and never received it? What's the best way to dispute this and get the additional fees removed? We're in CA.