system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do telemarketers, survey collectors, automated advertisers get my phone number ?

I'm bombarded daily with annoying calls from the aforementioned functions, despite telling them not to ever call back. How do they get my number? 

Edit: Thank you to everyone who contributed! You were all a huge help!
gpt
Companies you do business with sell your information, including phone number, email address and snail-mail address.

Put yourself on the FTC's "do not call" list and you should see the number of calls drop.  This doesn't prevent the overseas companies from calling you but it still helps.  Some people have even earned a few coins by suing telemarketers for contacting them after their names were put on the list.