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"#21#" is a GSM (Global System for Mobile Communications) code that allows you to check the call forwarding settings on your phone. When you enter "#21#" and press the call button on your mobile phone, it will display the current call forwarding settings for all types of calls, including voice calls, data calls, and fax calls. This code is often used to check whether call forwarding is activated on a mobile phone or to verify the forwarding number that has been set up. However, the specific response you get after dialing this code may vary depending on your mobile network operator and device.

Global System for Mobile Communication (GSM) and Unstructured Supplementary Service Data (USSD) are cell phone codes or smartphone codes that have been used since the early days of mobile telephones and cell phones.

Please see the pic below. On my iPhone 12 Pro iOS 16.4.1

There are some global USSD (Unstructured Supplementary Service Data) codes that work across multiple mobile network operators and countries. Here are a few examples:

1. *#06#: This code is used to check the IMEI (International Mobile Equipment Identity) number of your phone. The IMEI is a unique identifier assigned to each mobile device, and it can be useful for tracking your phone if it's lost or stolen.
2. *#31#: This code is used to hide your phone number when making outgoing calls. If you enter this code followed by the phone number you want to call, your caller ID will be hidden from the recipient.
3. *#100#: This code is used to check your account balance on some mobile networks. If you enter this code and press the call button, you should receive a text message with your current balance.

However, it's important to note that not all USSD codes work across all mobile networks or in all countries. The specific codes and their functions may vary depending on your location and service provider. It's always a good idea to check with your mobile network operator or consult your device manual to find out which USSD codes are available to you.

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