That fact nonwithstanding, users might be better served by having more "freedom of expression", e.g., by software that lets the user type any text into any field.

Of course, if the user doesn't succeed in entering a phone number when asked for a phone number, the user will probably need to confront that fact eventually (or fail to achieve what he or she set out to achieve) but it doesn't have to be during the typing into the field.

In fact, that is how web forms worked in the early and mid 1990s IIRC, and I have second-hand information that it was easy to get random users to interact productively with a web form during those years compared to how easy it was to get them to interact productively with other UIs, e.g., a form implemented in Windows 95.