SIM swapping: when your phone number turns into a security risk

A SIM card (Subscriber Identity Module) is a small chip that is inserted into your smartphone. It contains your phone number and important information such as contacts, network settings and messages. Once you sign a mobile phone contract and the card is activated, you can make phone calls, receive text messages, and use mobile data.

With SIM swapping, scammers pretend to be you to the mobile phone provider. They request a duplicate of your SIM card – and receive it. Your real card will be deactivated and the new one will end up in the attacker's phone. This allows them to take over your phone number and log in to services that use SMS for verification – such as banks, social networks or email providers.

Once they have access to your number, they can reset passwords, intercept two-factor codes, and even conduct transactions. In many cases, they use previously stolen personal data, for example through phishing, to make themselves appear credible.

To understand how SIM swapping works, it is helpful to know when mobile providers actually issue a new SIM card. In certain cases, a duplicate is necessary – for example, if the SIM card is damaged, the phone is lost, or a new device requires a different card size. As a rule, the old card is deactivated as soon as the new one is activated. However, some providers also allow you to use the same number on multiple devices at the same time, such as a smartphone, tablet or in the car.

Before issuing a duplicate, mobile phone providers usually require identity verification – for example, by presenting an ID card or using a customer password. This step is intended to ensure that only the authorized person has access to the phone number. But this is exactly where fraudsters start with SIM swapping: They try to circumvent or deceive these protection mechanisms – for example by using forged documents or stolen personal data.

A typical sign that your phone has been affected is that it suddenly no longer has a connection: You can no longer make phone calls, receive text messages, or have access to mobile data. Often, this only becomes apparent hours later – for example, when you are using Wi-Fi or your phone is on silent.

Other warning signs include:

• Your network operator or signal strength is no longer displayed.
• Calls are interrupted or cannot be made.
• You receive text messages with bank codes even though you haven't requested anything.
• New, unknown text or advertising messages appear.
• Your mobile phone bill suddenly includes additional costs or new SIM cards.

In some cases, scammers even demand a second active SIM card without deactivating the original one – this way they can use your number at the same time as another number.

 

If you suspect that you have been the victim of a SIM swapping attack, you should act immediately:

• Contact your mobile provider and have the fraudulent SIM card blocked. Request a new card to get your number back.
• Report the incident to the police, especially if financial damage has occurred or if accounts have been taken over.
• Inform your bank and have your cards and online access blocked. Please review all your activities.
• Secure your online accounts, especially those where your phone number is used to recover your password or login. Change your passwords and enable additional security features.

 

Even if SIM swapping is difficult to recognize, you can protect yourself with a few simple measures:

• Find out from your provider what the process is for issuing a replacement SIM. This way you know what data an attacker would need.
• Protect your personal data from phishing, smishing and other types of fraud.
• Avoid making your phone number publicly visible, such as in social networks or forums.
• Destroy sensitive documents before disposing of them – especially invoices or receipts containing personal data.

 

SIM swapping is a serious scam. Those who understand how this attack works can recognize warning signals more quickly and respond appropriately in an emergency.

By using simple protective measures – such as carefully handling your personal data, safeguarding your online accounts and a keeping a watchful eye for unusual activities – you can significantly reduce the risk. Vigilance and information are the best protection against it.