Is this “Debt Collector” legal? They will not provide any information.

A person received calls from an unknown number claiming to be a debt collector, stating they owed a loan to CC Bank through Berkley Processing Group. Despite verifying the debt's existence, red flags emerged, including demands for full payment without documentation, refusal to provide company details, and inconsistencies in communication methods. The individual confirmed the debt was sold to Berkley by American Debt Management but remains uncertain if the collector is legally legitimate or a scam.