Web Development

I actually heavily disagree. One of the WORST things you can do for user experience is defy their expectations for input. E.G...

• Users expect to have characters appear, where the cursor is, when they press character keys
• Users expect to have the character in front of the cursor be removed, when they press [Delete]
• Users expect to have the character behind the cursor be removed, when they press [Backspace]
• Users expect to have some kind of submission action take place, when they press [Enter]
• Users expect to be able to click on things that look clickable
• Users expect to not have elements on the page move underneath their fingers (I.E. as they type or click things)
• Users expect the contents of their clipboard to be pasted at the cursor position, when they perform a Paste

Breaking these rules by masking certain inputs generally makes them feel HORRIBLE to use.

Banks, for example, love to do this. I hit [1], [0], and [0] on the balance transfer form, to transfer $100, and I end up with a transfer for $1.00, because the validation forces a decimal point to be injected.

I think there are niche cases where keystroke rejection can make sense, but it's basically never worth the effort. And phone numbers is NOT one of them. When I buy stuff online, and get asked for a phone number, I'm virtually always pasting it in, because the only number I give out on the internet is a throwaway, and I don't have it memorized. More often than not, it gets rejected, because it was copied as "(XXX) XXX-XXXX", and the form doesn't like the formatting, even when that formatting is auto-applied anyway, as you type.

Good form validation involves letting users enter whatever the hell they want (within a max length restriction), and giving good feedback when it's not acceptable, indicating what needs to be corrected. Also, auto-completion goes a long way, when possible.