I see. Phone type fields DO default to the iOS numpad. Maybe this is what I was remembering. Go figure!!
As an alternative, I just tested using a Numpad app downloaded from the AppStore. When in the keyboard on the device, you do have to tap one more button to get to the Numpad. And later will need to switch back for text fields. Maybe that is an option? See image below
So, defaulting to numpad is possible and is probably an excellent item to submit as a Feature Idea - if not already on the list. I can see where some app creators would want the option to default to a QWERTY (or similar keyboard) while others default to numpad for numeric fields, so an app setting might be best approach.
After installing Numpad app and tapping keyboard option button
