I naively installed the app on my phone, which gave them my phone number. Then I started getting cold calls from people who paid to have access to my number through LinkedIn. I never entered my number anywhere, however the cold callers repeatedly told me they got my information from LinkedIn.
To be fair, they could just be saying LinkedIn because it's easier than saying they bought the number from some shady middlemen, and people are less likely to react negatively to it. Unless you used a unique number only for LinkedIn, the number could have been shared by anyone (even your telco).
What's the difference?