My mother has no idea how to call me from an android phone via phone, and instead always calls via Whatsapp.

As to reiterate that I have the reverse problem, like my mother, too:

As someone who has an iPhone and Whatsapp, often I fail to find a button to call people, and use whatever app I am currently using instead, even if that annoys me. But the UI is to complex and phoning people is a hidden/dark pattern in said apps. Or in Signal, I have contacts without phone numbers, so there is no possibility to call anyhow.