As some have rightly complained, once you set a Mask Input pattern on a Forms 2.0 field, the Form Editor doesn't let you switch the pattern based on other fields' values — for example, country-specific phone numbers.
(File this under “Things I've never needed, but know how to do.” Not a fan of input masks on web forms in general!)
Here's how. First, in Form Editor, give the field a default Mask:
Then the rest is done with a little Forms 2.0 JS…