Turn on your antivirus when you are registering on websites or downloading something. You could catch malware or something else, which leaked your phone number to the internet (or creator, and then he leaked the data). Or just use online mobile numbers in order to receive verification codes.
Personally, I am using phone lookup when I receive strange calls. And yes, I do not answer first.
Firstly, I check, and only then I am calling back. This tactic may save you from scammers and spammers. So be careful.