Unstructured Supplementary Service Data - Wikipedia
https://en.wikipedia.org/wiki/Unstructured_Supplementary_Service_Data
Unstructured Supplementary Service Data (USSD), sometimes referred to as "quick codes" or "feature codes", is a communications protocol used by GSM cellular telephones to communicate with the mobile network operator's computers.
What is USSD - MMI Code - USSD Code - How USSD Works ?
https://www.cspsprotocol.com/ussd/
USSD full form is Unstructured Supplementary Service Data. A mobile operator offers an essential service, along with voice and SMS. It does not require the internet connection from the device.