Well, in Zimbabwe because of the large user base WhatsApp got when it initially arrived (due to reduced charges, SMSs where about 15cents a pop then) around 2011-2012, the ISPs have effectively made it the only way to communicate economically by providing a "bundle".

Basically they offer you a drastically reduced price to only access WhatsApp, mostly text message without media at that. Compared to the data plan that can access the whole internet it is very very cheap to use just WhatsApp.

The cheapest WhatsApp bundle on the most popular network (Econet) is currently at around 51cents to text the whole week (65MB of data) vs the $13 for 8gigs. Granted the 8gigs is for a month but $13 is a big amount in this part of the world, and with regular data there's the aspect of discipline to not use all that data on youtube and other data hungry apps/websites.

How does this technically work? Does Econet use a dns-based filter?

You can actually send a message without saving a contact. Using WhatsApp's click to chat:

  wa.me/263xxxxxxxxx?text=Hello

Note: The phone number with country code and no preceding plus.

Opening that link will launch WhatsApp (or WhatsApp web) to the user's number and the text "Hello" in the message field.

On Chrome Desktop Linux, it doesn't work despite me being logged into whatsapp web. Tries to do some xdg-open thing, but clearly that will never work considering whatsapp web has no registered protocol handlers...

Looks like it is some half completed demo integration rather than production ready...

Send the link to yourself in whatsapp and then click on it.

Open a chat with yourself in whatsapp. https://wa.me/your-number . You can spam anything here, it's personal and only you can see it.

They send wa.me/number-of-person-you-are-contacting in that chat. Click on it and it will start a chat with person-you-are-contacting.

All numbers MUST be in international format. Country code and number.

I made a tool exactly to do this: https://trianguloy.github.io/OpenInWhatsapp_Web/

Yeah it used to be quite hopeless unless you opened the link in a tab that had already loaded WhatsApp web. Now however, if you click the green button that says "CONTINUE TO CHAT" on the initial screen, it'll load a page with an option to download WhatsApp and another that's titled "use WhatsApp Web", just click on that one. Unfortunately, it will load a new instance of WhatsApp web in the current window even if you already have another open in another tab.

Doesnt seems to work for me. Get directed to a page saying I don't have WhatsApp installed (I do).

Android v8, Firefox v68.6.

That is because you are opening in browser. You have to set wa.me links to open in app.

Open firefox settings, toggle 'open links in app'.

(or)

Open a chat with yourself in whatsapp. https://wa.me/your-number . You can spam anything here, it's personal and only you can see it.

They send wa.me/number-of-person-you-are-contacting in that chat. Click on it and it will start a chat with person-you-are-contacting.

All numbers MUST be in international format. Country code and number.

Reminds me of https://xkcd.com/1172/