There are hundreds of data brokers selling this type of info. An attacker who has a few minutes and a few dollars to spare will be able to acquire the average person's phone number. The underlying sources vary a lot.

Look up your own name or your friends on a site that is owned by or uses Intelius data, e.g. https://www.addresses.com/ - For many people (not all), their phone numbers come up right away.

Maybe I just have better data hygiene than average, but I found that site to be absolutely hilarious. I was able to find myself, but my address was several years and several moves out of date. My phone number was decades out of date and not even really my phone number. (It was a landline phone at my mom's house, with an old pre-area code split area code.)

Now try familytreenow.com for free or spokeo.com if you want to waste a dollar on a trial.

Pretty sure they won't be wrong or out of date.

The first one is indeed much closer for me. Still doesn't have my current phone number and while the broad strokes are there the dates are wildly inaccurate. Looking at my immediate family though, I actually seem to have the worst hygiene! It's 0/5 in providing an accurate phone number for any of us.

It's cause this info is consolidated from so many different databases, but it's out there. Some of the paid ones are scarily accurate.

One of those data breaches we hear about all the time