None cannot be a phone number, it’s not numbers.

maxspeed=walk is also not a number, or RO:urban, while „555-BARN“ is apparently a phonenumber. It’s called vanity numbers.

I’m perfectly ok with adding these specifics IF they are applicable, but the absence of the tag should be sufficient when there is no such number, a negative confirmation would rather clutter things more than it would be helpful to have a confirmation that at some time they really did not have it.