I tried finding older topics, but apparently mask has been removed since then? It still shows up in the documentation, but it is clearly no longer an option in the product. Is that correct? Is there any field masking at all anymore?
Correct - to my knowledge at least. I have a custom method I use to format phone numbers that you may be able to make use of with the -OnChange event handler:
Usage: Format-PhoneNumber -Number "2125550123"
Returns: +1 (212) 555-0123
DISCLAIMER: I have no earthly idea how this will work when using the OnChange handler if the value gets fed back into itself. Be careful!