I will openly ask the question as to whether there are commercially available sets of contacts for you that they've acquired. Did you have them as FB, MySpace, or any other social network contacts ever? Or have you ever (if you can remember) installed any apps that had access to your contacts that long ago? Any company that acquired that relationship between you and these people may have been acquired or gone out of business and sold this info as an asset, all within their TOS.

That person has a TikTok account and they still have you in their contacts list (and they have it synced)

Source though? I'm really curious if that's true. It's really hard to imagine some of these people saving my number; we haven't spoken in 10 years and barely knew each other to begin with.

How Facebook knows who all your friends are, even better than you do

(replace FB with your favourite social network)

TLDR;

In a nutshell, whenever you sign up for a Facebook account, Facebook asks permission to look at your e-mail contacts if you’re on a computer, or your phone contacts if you’re on a smartphone. When you grant the site permission, it searches your contacts for users already on the network, and it searches other users’ uploaded contacts for you.

https://www.washingtonpost.com/news/the-intersect/wp/2015/04...

How can you be sure that phone number remained the same if you have deleted it a long time ago?