Should be easy – SMS it's the "default" messaging app from a phone, isn't it?

How would my mom find it?

She knows that the green Whatsapp logo is for talking to her friends.

She probably wouldn't click the envelope looking thing.

(I'm not being facetious - IMO HN readers massively overestimate user capability.)

I agree completely. My mom still doesn't really understand the difference between phone calls, calls over Facebook Messenger, and calls over FaceTime

@ThePadawan Got it now! WhatsApp did a great job in tricking users with their interface. No other pun intended, but it fits perfectly in the context: "What's the App?"

I remember I missed a few calls from my mom recently, and she asked me if I noticed her missed calls. I didn't have any missed calls as she was calling me through WhatsApp, but in such a way that she thought is the "regular" way of calling. I explained, now she knows. Not sure where this is going...