Well there was a guy in the UK who set up a premium number after getting sick of cold sales calls. All companies get his 10p/minute number as contact (he gets 7p of that). Only friends and family get the real underlying number.
There are few available phone numbers which aren't already on numerous telemarketing / robospam systems. Exhaustive wardialing is highly tractable.
I've had numerous recent mobile and/or office numbers beseiged by dunning and marketing calls, apparently aimed at earlier holders (if even targeted at all).
https://www.bbc.co.uk/news/technology-23869462