I had a month to month agreement with Rain, I couldn’t connect to the internet and called them several times for assistance

I had a month to month agreement with Rain, I couldn’t connect to the internet and called them several times for assistance. I then decided to cancel the deal because it wasn’t working for me as a customer. About two months ago, they collected their device from me but they sold my contact information to a debt collector company that keeps on harassing me. I made several calls to Rain to have this sorted and they (Rain) confirmed many times that I do NOT owe them anything but this company Nudebt has my details calling me everyday requesting money from me. What money?