[wruza]

Correctly? Try explaining your grandparent that they should open the app and type in some codes while on call. This habit will expose them to a whole class of attacks.

The only proper way is to send push to that app with the information about the issue.

[pests]

They would also offer to hang up and when the person finally found the official number and called back, that same code could be given back over the phone to reconnect to the original agent. Or they could go through whatever process they want.