The service is possible:

If you are a mobile network operator.

Or, you can convince people to install something on their phone that sends you their location along with their phone number.

How would you go about reliably providing the location of someone's mobile phone without being their cell phone carrier?

By partnering with said cell phone carriers.

But I hope it would be illegal.

If movies have taught us anything, they need to remain on the call for at least 30 seconds for it to be traced.

How would this work?

If a phone number is for a mobile phone then looking up the location doesn't make sense at all: mobile phones are mobile.

I guess you could try and crawl an index of business phone numbers and associate those with the listed address for businesses, but that's a completely different business from running a geocoder.

You could provide a bit of geographical information about the first three digits of a US phone number. I imagine that's not what users are actually looking for though.

Phone numbers have geographic structure. For mobile phones it's just the area code, but for landlines there is also information in the exchange portion. For example, I grew up in Medford MA which is 781-39x-xxxx.

I expect there are also patterns in other countries?

Yeah, but you can probably use https://github.com/google/libphonenumber for that.

Network portability. While there can be a small amount of correlation to the entire zipcode, that quickly breaks down in portal number blocks and they can end up anywhere in the US.

Also, with VOIP it could reference where a number doesn't have long distance fees, but has nothing to do with where it is.

> I expect there are also patterns in other countries?

No, I believe this concept of mobile phone numbers being tied to localities is specific just to a few countries - USA, Canada and I think South Africa - due to how local calls were billed there; and for all other countries the mobile numbers were allocated from a single country-wide pool so their mobile phone numbers have no geographic structure other than the country code.

What if there were a service where anyone can opt in to share their own location information? If there was a way to get across the hurdle of how does the service provider know if the person is indeed who they claim to be…

My thought is something like a phone number, email address, or domain name bypasses this problem. It is fairly trivial to verify you have access to a phone number, email address, domain name. Feels like this is one of those chicken and egg problems though. Why would anyone list their location at my service if nobody queries it and why would anyone query my service if nobody lists their information with me?

Not in any European country that I know of. For example, the UK's geographical codes start with 01 and 02 (020 is London, 0238 is Southampton, 02820 is Ballycastle) and are never assigned to mobile phone numbers, which always start with 07.

Other countries work in much the same way, except the prefixes are, of course, different.

This is what the inaccurate youtube tutorials do that they complain about in the linked blog post. It only gets you country level lookups for most phone numbers, county level at the very best. And it turns out users don't want that.

There's also the issue of moving. I have a Florida phone number. I haven't lived in Florida in a long time.

You mean if they could figure out how to illegally track millions of people?

> a lot of people would be interested in this

you mean like scammers and stalkers? (ok, and probably Meta)