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There is no requirement that a consumer be notified before a debt collector can report their collection.
As for a dispute, first, the creditor had the right, at any time, to hire a debt collector for collection assistance.
Once the debt collector has legitimate collection authority, they are entitled to report their collection to the CRAs.
Reporting of a collection has been held, under case law, to then trigger the requirment of the debt collector to send dunning notice within 5 days, but notice is not required to be provided to the consumer prior to reporting.
I see no inaccuracy in the assignment to the debt collector, or in the reporting of the collection.
Thus, there is no basis for a dispute on those grounds.
You need to work with TMobile to have the debt recalled from collections. Try going through the Executive Office, the CEO seems willing as of late to improve their public image regarding prior poor customer service.