Replying to OP because it's the most logical place to put this - I'm curious what a "USSD code" is, and what kinds of codes I could input into the dialpad that

Replying to OP because it's the most logical place to put this - I'm curious what a "USSD code" is, and what kinds of codes I could input into the dialpad that do interesting things. (I'm already aware of standard telco features like call forwarding, and I know iOS and Android both have their own set of "easter eggs" accessible via the dialer. I'm talking specifically about non-secret codes that talk to the baseband and stuff like that, if this sort of thing exists.)