David Crookes, Tom’s Guide:

For more than 20 years, mobile devices have made use of Unstructured Supplementary Service Data (USSD) codes. They look like a series of numbers interspersed with hashtags and asterisks that you enter like a telephone number.

Secret codes seems a bit of a reach. But these USSD codes are definitely worth knowing about. Scan the post, just to get a sense of the functionality you can access via your dialer.