How does Mr. Number and Number Guru do it? I don't know, but if they find a number is 'not in existence' they report it as so. Maybe they have someone assigned to dial the numbers that are reported to them, and then document what they find if it comes back as 'cannot be completed as dialed.'
Saves us from having to call it to find that out, which is dangerous in this day and age.
I just checked numberguru.com. I plugged this number in, and it said nothing about it being a number "not in existence". In any case, those are also phone apps, which is not the case for 800notes. If those apps have the features that you want, then use them.
Called and I amswered. They didn't say anything and hung up after 10 seconds.
Called back and it's a Google number. This means it was made from a computer or a phone thru the Google phone service. In other words anyone can use that survice to hide their identity and show this phone number instead. The number is to a computer located somewhere in LA.