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We are using Authy for phone number verification on our website. It is very easy to create a free online SMS account through Twilio or other providers.

Is there any way to determine whether the phone number is tied to a genuine SIM card and not from an online SMS account?

I know that individuals can still buy a load of SIM cards, but being able to check this aspect during verification would improve things.

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Most VoIP system tries to behave like a real phone, so unless you have access to the internals of a telco, you can never be sure what kind of device is behind a number.

One alternative is to make the user call a special number (premium, toll-free, etc.) that requires contracts to be setup to call, then you can control exactly which telcos you'd setup the contracts with.

Similarly, you can establish ties with major telcos so they can tell you if the number belongs to a subscriber.

Another unsupported ways is finding SMS gateway addresses of popular carriers and asking the user to select the carrier to receive the text. Only numbers that are actually with the carrier can receive the text.

Also, if you can write an app to be installed on the user's phone, you can access their SIM info.

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