Thanks to modern digital technology, it's less possible than it once was to clone a cellphone simply by monitoring signals between the phone and the tower. Unfortunately, it's still possible to steal someone's phone number by defrauding a phone company, and by the time the actual user says "someone cloned my phone number," the fraudster can use the phone to log in to various services and intercept calls and texts. Cloned phone symptoms include error messages and issues receiving calls and texts.
Traditional Cellphone Cloning
Traditionally, cellphones were cloned by monitoring signals between phone company towers and individual phones to grab the identifiers of the phones. Fraudsters would then program their own phones to use the stolen identifiers and use them to make expensive calls, such as international long distance calls, on someone else's bill.
That's gotten a lot harder thanks to encrypted digital signaling, though it can still happen occasionally where phones fall back to an analog signal in particular areas.
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More modern devices known as IMSI catchers are used by law enforcement and shadier people to impersonate cellphone towers and grab a phone's serial number, typically known as the IMSI, or international mobile subscriber identity. This can be used to spot where a phone and, by extension, its owner go, but it's usually not enough to duplicate the phone on the network.
Modern Cellphone Cloning
More commonly today, someone will duplicate a phone by impersonating its owner to the phone carrier, asking that the number be ported to a new phone. These scammers often aren't after cheap calls, especially now that there are so many ways to speak to people internationally over the internet.
Instead, they're looking to use the phone number to reset passwords on other services, from email addresses to bank accounts.
To make this less a risk, you can make sure you have a password or PIN set with your carrier before your phone number can be moved to a new phone. You can also avoid using phone number-based authentication in favor of alternatives, such as apps that generate temporary login codes, where you can.
Recognizing Cloned Phone Symptoms
If someone has cloned your phone, you may get error messages that your phone can't connect to the network and you may miss calls and texts, since they're being routed to the other phone using your number.
You may also receive some notification from your cellphone company by text or email that your number has been moved to a new device.
If the phone is being used to get access to other apps or services, you might get notifications from those companies that you've signed in on a new device. If it's being used to make calls, you'll see them on your phone bill.