May 8, 2026, 6:02 am | Read time: 2 minutes
Many smartphone users don’t notice when their device is being monitored. However, there are simple ways to detect tampering. Special secret codes, entered directly through the phone app, provide clues about unauthorized forwarding and suspicious settings. Knowing them allows for quick action to close potential security gaps.
Secret Codes Reveal Possible Tampering
Secret surveillance on smartphones is not uncommon and often goes undetected for a long time. However, with the help of so-called USSD codes, you can check if a device has been tampered with. These codes are entered like a phone number in the phone app and executed by tapping the call button.
Among the most important check codes is *#21#, which shows if calls or SMS are being forwarded without your knowledge. With ##21#, such forwarding can be deactivated. The code *#67# checks forwarding that occurs when the device is unreachable. Regular voicemail forwarding may also appear. With ##67#, these can be turned off.
The code ##002# immediately deactivates all forwarding. Information on forwarding when the line is busy is provided by *#004#, while ##004# deactivates this function. *#33# opens call barring settings, such as blocking unwanted calls. *#06# displays the device’s unique IMEI number.
Depending on the device, additional codes are available. On iPhones, 3001#12345# displays detailed network and connection information. Android users can get similar data with ##4636##. For Samsung devices, *#197328640# provides a comparable overview.
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Further Signs of a Monitored Smartphone
The mentioned codes primarily uncover tampering with call forwarding. Modern spy software, however, often remains hidden. Therefore, using a security app can be useful to identify malware and determine if a smartphone is being monitored.
In addition to the codes, there are other signs that may indicate a possible hack. These include a suddenly faster draining battery, unusually high heat generation of the device, significantly increased data usage, or the appearance of unknown apps.
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Actions for Suspicious Results
If a code shows an unknown forwarding, it should be deactivated immediately, for example with ##21# or ##67#. Additionally, it is advisable to change all important passwords, as they may have been accessed.
Furthermore, you should install a security app and check the device. If doubts persist, it may be wise to reset the smartphone to factory settings.