BMW Financial Services - BAD AFTER SERVICE

I sold my BMW which was originally financed by BMW Financial Services. The vehicle was settled by the dealership last week. BMW has been harassing me with debt collecter calls stating that I haven't paid the account. I sent them the POP multiple times, but they don't come to confirm. They literally just send me emails via their collector threatening to put in a judgement against me. Yet they can't respond and the consultants don't know what is going one.