It’s not that people believe this - it’s just that they don’t think things through when building something. This point was mention because there are plenty of CRM/contact-management systems that only have a single phone number field per contact. Or worse: Outlook, which is expressly marketed as a contact management tool, with its arbitrary restriction of 3 phone numbers and 3 e-mail addresses per contact - or how all contacts have to have first and last names (how do I have a “company” contact? Etc). Point is, don’t be like Outlook.

I think it's hyperbole for "things you should be careful about". Obviously nobody really believes every human on the planet has a phone number, or only one, or that there don't exist people who share phone numbers... or pretty much every other item in this list :P

I think it was a riff on the "falsehoods programmers believe about names" article, which is a bit more insightful.

>> An individual has only one phone number

> Always feel these lists include things no human actually believes.

Well, somebody believes that. I can register my phone number with the local postal system, and, as shippers add a phone number to shipments, I can search for shipments to me by logging in to my post company account on the net, or via the Android app. The problem? I can only register a single phone number. But I have two numbers, and, for various reasons, different shippers must sometimes use one or the other, and not the same number. But I can't register more than one, and I can't register two accounts - b/c they also need my social security number, which is unique. So I can only track a subset of shipments to me. All because somebody there (it's their own software) made the assumption that an individual can only have, or at least use, a single phone number. I did report this as a bug, with no understanding.

Programmers make stupid assumptions all the time... usually because they are focused on some other more difficult problem and they assume some other problem like this is easy and they don't have to think much abt it, so they don't.