[imadethis]

I wonder how many websites / applications would break or error if a user tried to enter a valid 555 number.

[Splatter]

I have a valid phone number that ends in 0000 that I got multiple decades ago.

There are many websites where I enter my phone number and it's rejected. Sometimes programmers are too clever for their own good.

[PenguinCoder]

A few do that I've tried signing up for services with. Not a lot though.