Usually they cold call or email with prices/quotes/offers(see my previous comment to parent), and somehow they harvest of buy the contacts of businesses and employees.

I sometimes suspect that, there are some ways to collect these from linkedin or the business card printers sell the contact info in black(due to strict data privacy act in EU). Because only two places my work email & work phone number being available are at the business card printer and linkedin(we need to use work email to access some elearning things, don’t ask).