It is also possible to communicate without using Meta services.

> it is now possible to install more than one app on a phone at a time.

And, in doing so, achieve the security posture of the worse of the apps!

You realise you could use that phone number to...call them and let them know? Also - in what situation are they giving you a phone number and telling you they use WhatsApp but you having no way to respond when receiving that info? If it's in person you can explain at the time. If it's taken from a website, call them. Or you can even fall back on SMS.

Presumably at the time they've given you their phone number, they've told you that they are on WhatsApp, and then you've responded directly that you're only on Signal.

If there is a communications channel by which they can give you their phone number, you can use that same channel to discuss what messenger to use.

If they can't reach you via whatsapp, they will call you.

Sounds like someone who never had to pay for each SMS.