As several people have commented, it's common on phone conferencing systems, when entering the conference code "followed by the pound sign". It's also used to

As several people have commented, it's common on phone conferencing systems, when entering the conference code "followed by the pound sign". It's also used to check Verizon FiOS voice mail, where you enter it after entering your PIN. As for the *, up until recently, I had a voice mail system where you used it if you were checking messages from a phone other than your own, to signal you'd be entering a phone (non-menu) number.