Contact information should belong to the person it is about.
Every address book ever built rests on the same assumption: that a phone number is a thing you own a copy of. You write it down, and from that moment it is yours to maintain. When it goes wrong — and it does, constantly, quietly — that is your problem to notice and your problem to fix.
It is a strange arrangement when you look at it directly. Zoe knows her own number. She knew the day it changed. The only person who has to find out the hard way is you.
Tether starts from the other end. You do not hold a copy of Zoe's number — you hold a connection to Zoe, and her number is one of the things that connection carries. She changes it once, and everyone who should know, knows. Nobody sends a group text. Nobody has to ask.
That only works if it is genuinely safe, so the permission model came first, not last. You decide what each circle of people can see about you, you can look at your own card through anyone's eyes to check, and when you stop sharing something it is actually gone — no stale copy sitting on someone else's phone.
And it only works if people are actually on it, which is why the free tier is the product rather than a trial. We would rather you join than pay.
The rest of it
Tether is built by a very small team in the United States. There are no ads, no ad-tech trackers, and no data sale — the business is subscriptions, and that is deliberately the only thing it is.