Everyone knows your location: tracking myself down through in-app ads

One big privacy issue is that there is no sane way to protect your contact details from being sold, regardless of what you do.

As soon as your cousin clicks "Yes, I would like to share the entire contents of my contacts with you" when they launch TikTok your name, phone number, email etc are all in the crowd.

And I buy this stuff. Every time I need customer service and I'm getting stonewalled I just go onto a marketplace, find an exec and buy their details for pennies and call them up on their cellphone. (this is usually successful, but can backfire badly -- CashApp terminated my account for this shenanigans)

The thing is...contact details aren't really private information, basically by definition.

The distinction is contact details privacy is based on the desire not be interrupted by people you didn't agree to be interrupted by - i.e. it's a spam problem - and realistically to solve this requires a total revamp of our communications systems (long overdue).

The basic level of this would be forcing businesses to positively identify themselves to contact people - i.e. we need TLS certificates on voice calls, tied to government issued business identifiers. That would have the highest immediate impact, because we could retrain people not to talk to anyone claiming to be a business if there phone doesn't show a certificate - we already teach this for email, so the skill is becoming more widespread.

A more advanced version of this might be to get rid of the notion of fixed phone numbers entirely: i.e. sharing contacts is now just a cryptographic key exchange where I sign their public certificate which the cellphone infrastructure validates to agree to route a call to my device from their device (with some provisioning for chain of trust so a corporate entity can sign legally recognized bodies, but not say, transfer details around).

This would solve a pile of problems, including just business decommissioning - i.e. once a company shuts down, even if you scraped their database you wouldn't be able to use any of the contact information unless you had the hardware call origination gear + the telecom company still recognized the key.

Add an escrow system on top of this so "phone numbers" can still work - i.e. you can get a random number to give to people that will do a "trust on first use" thing, or "trust till revoked" thing (i.e. no one needs to give a fake number anymore, convention would be they're all fake numbers, but blocking the number would also not actually block anyone you still want to talk to).

EDIT: I've sort of inverted the technical vs practical details here I realize - i.e. if I were implementing this, the public marketing campaign would be "you can have as many phone numbers as you want" but your friends don't have to update if you change it. The UI ideally would be "block this contact and revoke this number?" on a phone which would be nice and unambiguous - possibly with a "send a new number to your friends?" option (in fact this could be 150 new numbers, one per friend since under the hood it would all be public key cryptography). I think people would understand this.

What definition of contact details makes them not private?

Contact details (your phone number, email or address) are definitively private information, you should be the one that decides who gets them and who doesn't.

Literally explained in the second paragraph there.

You can't have private information which is meant to also be shared widely. It is the distinction between Access and Authorization.

But it's not meant to be shared widely, for most people it's meant to be shared with consideration and/or permission.

Also, it's not just about "a desire not be interrupted by people you didn't agree to be interrupted by", it's about not having the data in the first place, for any reason, including tracking of any sorts.