Phone number checker
Rate limited, no key required. Results appear inline.
What the check returns
Valid, meaning correctly formatted and inside an allocated range for that country. Line type, which is the field worth reading: mobile, landline, VOIP or toll-free. Carrier and country, the network the number belongs to and where it is registered. Plus VOIP and disposable flags, and the number rewritten in canonical international format so two spellings of the same number do not become two records.
Nothing is sent to the number. No call, no message, no code. Everything here comes from numbering-plan data and lookups, which is why it is instant and free.
The VOIP question
Phone verification earned its reputation because a real number used to cost real money. One person, one SIM, one number. That assumption is what made “verify by SMS” a genuine barrier.
VOIP broke it. Numbers that route over the internet are cheap or free and can be created in bulk.
The useful framing is cost, not legitimacy. A VOIP number tells you this account was cheap to create. That matters enormously when what sits behind it is a signup bonus or a free trial, and much less when it is a paid subscription where the card is doing the work anyway.
So weigh it against the stake. The same VOIP number should sail through a checkout and get a second look at a cashout.
Line type is the useful field
Mobile is the strongest signal of a real person, because a SIM has a cost and an identity attached in most countries.
Landline is unremarkable and slightly odd on a consumer signup, since almost nobody gives a landline to a website any more. Worth noticing, not worth acting on.
VOIP is the one to weigh, as above. Toll-free on a consumer signup is close to meaningless, since it is a business number and no individual customer has one.
The pattern that actually costs money is not a single VOIP number. It is forty of them, sequential, from one carrier, arriving in an afternoon. That is a bonus abuse pattern and no single-number check will show it to you, which is why the number belongs in a score alongside the device and the connection.
What a phone number cannot tell you
Whether it is in service. Valid means correctly formatted and allocated. A number can be perfectly valid and disconnected for years.
Whether the person owns it. A number typed into your form is a claim. Only sending a code and having someone read it back turns that into evidence, and even then it proves access at that moment, not ownership.
Anything about porting. Numbers move between carriers, so the carrier field tells you who serves it today and nothing about its history. Carrier alone is a weak signal for the same reason.
Checking it at signup
The check above is one endpoint of the Kaidn API:
curl -X POST https://api.kaidn.io/v1/check/phone \ -H "x-api-key: $KAIDN_API_KEY" \ -H "content-type: application/json" \ -d '{"phone":"+1 415 555 0134"}' # { # "phone": { # "fraud_score": 55, # "valid": true, # "is_voip": true, # "is_disposable": false, # "line_type": "voip", # "country": "US", # "carrier": "Google Voice", # "formatted": "+1 415-555-0134" # }, # "summary": "Valid US number, but VOIP. Cheap to create in bulk." # }
The better pattern is POST /v1/score with the number alongside the email, the IP and the device. A VOIP number on a residential connection with a first-seen device is a Google Voice user. The same number on a datacenter IP with a device already tied to four accounts is a farm, and only the second should cost anyone a signup.
Checking is also cheaper than verifying. Knowing what kind of number it is before you spend money sending an SMS saves you the cost of texting numbers that were never going to convert.
Frequently asked questions
Frequently asked questions
Is this phone checker free?
Free, no signup. Paste a number in international format and read the result. Nothing is stored against you or the number, and no call or message is ever placed to it.
What is a VOIP number and why does it matter?
A number that routes over the internet rather than a carrier's mobile or landline network. It matters because VOIP numbers are cheap or free and can be created in bulk, so one person can hold hundreds. That is the opposite of the property that made phone verification useful in the first place, which was that a real number costs real money.
Should I block VOIP numbers?
Not outright. Google Voice is VOIP and it is used by millions of ordinary Americans as their main number, so a blanket block turns away real customers in your largest market. Treat it as a cost signal: a VOIP number means this account was cheap to create, which matters when the thing behind it is a signup bonus and matters much less when it is a paid subscription.
Does a valid number mean a real person?
No. Valid means the number is correctly formatted and sits in an allocated range for that country. It says nothing about whether the number is in service, whether anyone answers it, or whether the person filling in your form has any connection to it.
What format should I use?
International format with the country code, like +1 415 555 0134. Spaces and dashes are fine. Without a country code the number is ambiguous, because the same digits are valid in several countries and mean different places.
Do you send an SMS to verify the number?
No. Nothing is sent. Everything returned is derived from numbering-plan data, carrier and line-type lookups and reputation data. Sending a code and checking that someone can read it is a separate step, and a useful one, but it is verification rather than checking.
Is phone checking worth it if I already verify by SMS?
Yes, because they answer different questions. SMS verification proves someone can receive a message at that number right now. Checking tells you what kind of number it is before you spend money sending anything, which also saves you the cost of texting numbers that were never going to convert.
How do I check numbers at signup automatically?
Call POST /v1/check/phone from your server, or POST /v1/score to weigh the number alongside the email, the IP and the device and get one verdict back with the reasons behind it.
Run this on every signup
The check above is one endpoint. The same engine scores a whole event, the address, the connection and the device together, and returns one verdict with the reasons behind it.