BleepingComputer.com: Wait, what did I just do? USSD?

Hi all,

Earlier today when I was putting my phone down I accidentally hit a few buttons and heard it beep.

When I checked it, a box had popped up saying "USSD Finished" with the only option to hit OK.

I have... no idea what that means or what I did and I found a wikipedia entry for USSD but I don't really get it.

I have a feature phone, by the way, not a smartphone. I plan to call my phone provider and ask their support line but they are closed/unavailable for the next little bit.

Can someone help me out of the dark here?

Thank you.