Why your address book goes wrong
Think about what happens when you save someone’s number.
They tell you a string of digits. You type it into your phone. From that instant, you own a copy — and the copy and the truth are two separate things that will drift apart. Not might. Will. The only question is how long it takes and how you find out.
Usually you find out badly. You text a number you have had for six years and it does not land. Somebody mentions in passing that they moved last spring. A card comes back, or worse, does not come back and nobody ever mentions it.
The asymmetry nobody notices
Some of these failures announce themselves and some do not, and the difference matters more than it looks.
A wrong phone number is loud. The message bounces, the call fails, you notice, you ask, you fix it. The system self-corrects, badly and late, but it corrects.
A wrong address is completely silent. You send the card. It goes to a house where somebody else lives now. They throw it away. You do not find out this year, and you do not find out next year either, because there is no mechanism anywhere in the world that tells you a card did not arrive.
This is why the addresses in your phone are, on average, the oldest and most wrong data you own. Not because you are careless about addresses. Because addresses never tell you when they break.
It is not a discipline problem
The usual response to this is a personal one — be better organised, do an annual cleanup, keep on top of it. People have been advising that for as long as address books have existed, and address books are still wrong.
They are wrong because the design puts the work in the wrong place. The person who knows a number changed is the person whose number it is. They knew on the day. The person who has to discover it, somehow, is everybody else — dozens of people, independently, each finding out through their own small failure, each fixing their own private copy.
That is an enormous amount of redundant work to extract from the one fact that a single person already knew.
The other way round
So do not keep a copy.
Keep a connection to the person, and let their details be something that connection carries. They change their number once. Everyone who should know, knows. Nobody sends a group text. Nobody has to ask “hey, is this still you?”
That is the whole idea behind what we are building. Everything else — the permissions, the circles, the things you can decide to show one group and not another — exists to make that arrangement safe enough that people will actually agree to it.
Because they should get to decide. It is their number.