There's someone you don't recognise texting about the sofa you listed last week from a number you've never seen. It's tempting to reply to find out who they are, but first you should ask yourself, is that their phone? Here's what you can find out. And nothing you can find out will be definitive.
The digits prove less than you think
A US number can't tell you what kind of line it is. Because landlines, mobile lines and VoIP lines all share the same numbering plan, the only thing a 312 number tells you is that it was issued to a Chicago line. The line might be in Chicago.
It might be in Leeds, for all you know, because the system is entirely portable: you switch to the new network but keep the same digits. Porting in the US and UK makes this worse still. When you move your mobile or landline number between networks, you keep the same eleven digits, so in the UK a 07 number that started on a mobile network may be running on an internet platform today. Today, a UK 07 number is just as able to be running on a virtual platform as a genuine network. Similarly in the US, a 213 number issued by one carrier might have ported over to another network years ago.
Lookups, line types and empty results
Your next stop is a carrier lookup. These services check the number against the porting tables to reveal what type of line it is, and who is currently hosting it. In the US, porting records are stored in the NPAC, the Number Portability Administration Center, run by iconectiv since 2018. If the answer is "VoIP", the line exists on the internet, not on a phone network.
There's nothing else, however, it won't tell you who is using it. Reverse phone directories and people-search sites do their best to fill in the gaps by returning a name, address and social media profiles associated with the number. Type in a familiar, ten-year-old mobile and you'll probably get a name; type in a new virtual number and it's unlikely you'll get a match, but check your blank space, lots of people keep their telephone number off the internet.
The verified label, and how easy it is to fake
Under the FCC's STIR/SHAKEN set of rules, big US voice providers were required to sign calls by 30 June 2021. That was a result of the TRACED Act, which was signed into law on 30 December 2019. Signatures show the attestation level for calls, A for fully signed, B for partially signed and C for gateway, with a verified icon on handsets if signed along the way. A caller ID without a marker simply couldn't be signed throughout.
Some of this, it's worth having, and worth not believing. Caller ID can be spoofed entirely. The Truth in Caller ID Act of 2009 in the US makes it a crime to intentionally transmit misleading numbers with the aim of defrauding, causing harm or wrongfully obtaining something of value; this law is on the books because some people keep doing it. The number displayed might not be the caller's number. Dialling 1471 in the UK will display the number of your last caller (if they didn't withhold it), and 141 will withhold your own. Dialling *67 in the US will do the same for a single call.
Burner, app number, or a second line
'Burner' is a term that originated as slang for a low-cost prepaid mobile purchased with cash, used for a time and then disposed of. The HBO series The Wire (2002 to 2008) popularized the term. A burner phone number resides on the SIM chip inside the phone and can therefore move to another device; an app-based second number on a device doesn't have a SIM chip, just an app that rings on a cell phone that already has a number.
Messaging apps have theirs. On Telegram you can choose a hidden number, under Privacy and Security, then Phone Number, then Nobody and since December 2022 even open an account with an anonymous +888 number, bought on the Fragment platform, separate from a SIM card. The UK still has a pseudo-anonymous pay-as-you-go SIM, and Germany has maintained requiring an ID since July 2017.
Who can actually trace it
Your phone company won't give it. Provider-supplied subscriber info won't be forthcoming, no matter how nicely you ask. In the US, the police can obtain basic subscriber data through a subpoena using the 1986 Stored Communications Act; the UK's 2016 Investigatory Powers Act allows communications data searches by the authorities. The person able to look up that number isn't you; it's a cop with a folder.
The provider has the account and the transaction records. Mobile operators record the cell towers to which a SIM connects, so even a cash-purchased prepaid SIM will leave a record of the cell sites. No lookup site will recreate that for you.
MoreMins offers numbers to over 50 countries, with UK and US numbers starting at $0.99/month in an app without a SIM or contract. That's essentially a burner number - a second line that you can ditch without even touching your primary one.
The short version
The number on your screen won't inform you if the calling number is from a real mobile or a spoofed line, and there's no lookup site for that either. Line-type verification is only an indicator, at most. A provider - and the authorities, with legal process - can confirm.