...why would I give my phone number to random people to pay them? I'd rather use an email address or user name. And using phone numbers means making it easier for normal people which means uploading the whole address book to the bank... just no.
The person above you is right. I've never had it happen to anyone I know so the risk seems exceptionally low (also because of the undo option), but technically you can indeed just fill in any IBAN and get that person to pay for your product. Responding "but that's illegal!" is like wondering why we need police at all; nobody would violate the law right??