Unstructured Supplementary Service Data (USSD) is a communications protocol for mobile phones. Using USSD codes, you can access web-based services without installing a mobile app. Here's what it is, how it works, and why it matters.

The USSD messaging protocol is part of the Global System for Mobile Communications (GSM) digital mobile standard. Like SMS and MMS, USSD facilitates communication without the need for a special app. Unlike SMS, which is used for back-and-forth text messaging between two phones, USSD establishes a real-time connection between your phone and a mobile network or server. This allows you to do things like add time to your phone plan, check your bank account, and more, without having to download or install apps on your device.

USSD is based on codes that consist of characters that can be found on every mobile phone. When you dial a USSD code, your phone sends a request to a USSD network gateway, which routes the request to a web-based application. The network gateway then returns a text-based menu that you can use to interact with the app.

For example, on prepaid phones you can use USSD codes to check your remaining balance, enable roaming or use location-based services. It is also possible to receive social media updates from Facebook and X (formerly Twitter) via USSD. USSD codes can be used to transfer money, set up call forwarding and even surf the web via Wireless Application Protocol (WAP).