@bjonah What is USSD? Please provide the full name for that acronym so we have more context for your question.
Unstructured Supplementary Service Data, 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.
Perfect!. Thank you. I have never delved into GSM details. In fact I don't even carry a GSM phone; Verizon (my carrier) is the big CDMA / non-GSM holdout in the USA. Your quick explanation helps tremendously, especially in further educating me.