You can change your number
I never understood using phone # as a permanent ID.  phone numbers change (heck, I effectively have 2 whatsapps, because I have a US phone # and an international phone # because of this).ID shouldn't matter to most users (it can be hidden behind the scenes).  Phone # is great for looking up the ID, but users should be able to remap it at will.Example:register with your phone #.  This generates a new ID (you don't know or care about it).  If you have to login from a new device, that doesn't have the ID stored, can you login with your phone #, but all this does is look up the ID and uses that ID to try and then authenticate you.If someone wants to find you, they use the phone number to look up your ID.  Once its looked up and mapped, the phone number never needs to be used again.If I change my phone #, all I have to do is update the mapping of phone # -> id (i.e. add a new entry, remove the old entry).  Anyone contacts who have me already, will not be bothered by this (they only care about the ID, which they already have).  new "contacts" will also behave correctly, as I no longer have that phone #, so it shouldn't be able to be used to find me (it might be someone else's # now).Users would be able to move phone #s and their existing contacts would be able to follow them.  New telephone users would be able to get recycled old phone #s without getting messages from the old owner of number's contacts (assuming they had previously contacted).the only places I see people think this might fall down (but I think are wrong) is1) if the same user creates a new id with the old phone #.  However, the solution seems pretty simple, you just need a way to invalidate the old ID (i.e. never to be used again) and force the contact to get the new id for the phone number.2) what happens when a user moves devices.  i.e. they might have to redo the mapping of phone # -> id.  However. at its worst, this is no worse than the current system (which effectively does that update on every single message).  In practice, there are ways to move data between devices which would just move the mappings with it (examples being a cloud cache backup, the ability migrate data from device to device, or probably other ways as well).