This doesn't make a whole lot of sense because people give up their phone numbers from time to time. I did this myself. My understanding is that someone who claims my number cannot access my past data. But if it is "stateless" then a phone number doesn't have to be the only identifier. They can also use email. Heck .. here is an algorithm ... use phone number 555XXXXXXX .. take a person's email address and hash it to XXXXXXX. Done!

Most people in the world don't (or barely) use email, specially in EMs. Also, you can access you email from many different places, you (usually) can't receive an SMS on the same number from many different places.

Another advantage of using phone numbers is that people actually have other people's numbers in their phone address books, which whatsapp uses. Very few people have their friends' email addresses on their contacts.

I guess if the point is for them not to store your messages on their servers once those messages have been delivered, how is WhatsApp to know whether to send a message to the session logged in from your phone, or to the desktop app with a different session, or to your work computer where you forgot to log out?

Being able to sync across all your devices requires they start storing all of your communications centrally, which defeats the whole information security model.

Can't it be device to device , once you get to new device you mark that device as add and as soon as u login it can sync.

But i found whatsapp is next to mp3 in my and my friends phone so it is not easy from bandwidth perspective

This way, you might not have the full conversation on each device - e.g. you write "a" to alice on your phone, turn it off, turn your pc on and receive "what do you mean?" from alice - the conversation is otherwise empty. That's not a good UX. You'd have to store the chat history somehow - and thus loose the privacy aspect of not storing it.

Giving phone numbers up, typical in the US, doesn't happen everywhere in the world.

Whatsapp only needs one time authentication through SMS OTP. And whats app need not be installed in the same phone as that containing the number. I use number from a different country for my whatsapp. I dont even have the phone with me. So when the OTP is received, I ask my friend to provide it to me.

That doesn't explain them actively blocking google voice numbers.

Or home phone numbers (landlines) for that matter. Let people use a landline number on a tablet.

I've had it working perfectly well with my google voice number for years now. When did they start blocking it?

The only problem I recall is that the initial text verification doesn't work - I had to do the phone call option - but this is a problem with gvoice, not WhatsApp: google blocks pretty much all server-triggered texts.

I ported my Cingular/AT&T number to Google Voice many years ago and six months ago ported it to Ting. There are a number of companies who consistently fail to deliver texts to that number. The most notable (for me) is Amazon. Yet dozens of other companies (Facebook, Google, etc.) are able to use it just fine.

I don't have the slightest idea on how to fix it. The only reason it worries me a bit is that I might miss texts from other people.

Switched to an iPhone so I could use "Find Friends" with spouse and kids.

Yup, works for me as well!

Telegram also relies on phone numbers, and you need them to just to log in. You can log in once and stay logged forever on your computer without ever touching your phone again.

Whatsapp makes you to stay connected at the same time on your phone to use their web/desktop apps.

WhatsApp doesn't store messages on their servers, which is one of those features I really like in a privacy point of view. Signal does the same, its web app only loads the messages from your phone and they are not stored to any servers.

Doesn't that mean you can't carry on a conversation on another device?

It seems that was the main reason people didn't like Jabber to start with.

Still, Signal's web app works without a phone AFAIK