"Unlisted" does not equal "invisible" or "unreachable". Every autodialer has the potential capacity to dial every valid ten digit combination in numeric sequence. Fraudulent sales callers tend to rely on far more focused lead lists, which may become populated and reinforced every time your phone number is associated with a commercial transaction. Heavily guarded phone numbers have a lower "street value". If you instead give them away to your grocers, retailers, lenders, surveyors, job headhunters, and social media, you can expect your numbers to be targeted and abused. Debt collectors of course begin working from a much smaller number pool as supplied in debt portfolio records and credit applications.