I'd like to take the opportunity to mention a tiny very useful app that allows opening a WhatsApp chat directly with any number, without having to register it first as a contact. Great for vacations or similar situations where a quick one-time chat is needed with somebody:

* https://play.google.com/store/apps/details?id=com.trianguloy...

* Webapp: https://trianguloy.github.io/OpenInWhatsapp_Web/

I'm just grateful for this app, so I thought that maybe other HNers might find it useful.