It's called USSD, there's a short list on Wikipedia. A lot of them are carrier dependant, and some carrier models of certain handsets will block some codes.

https://en.wikipedia.org/wiki/Unstructured_Supplementary_Ser...

Not the OP, but I remember these codes from my first phone, a Nokia 5110.

In addition to the already mentioned code of [STAR]3001#12345# there is also [STAR]#06# which will show your IMEI.

There were plenty of others (including spelling out 'WARRANTY' to get the manufacture date of the phone) but I forgot most of them.

Interestingly, on the iPhone, dialling anything of the format [STAR]# ...anything... # will cause the screen to go grey as if a separate app, then after a short delay tell you that your code was invalid.