If you don't use the operator extension, you could use that, forwarded to an IVR that then has an option to go to 999. You don't have to announce this option. Because this would be reached by *, you could simply offer someone who "stumbled" on this, an option to leave a message, by pressing 1, but if you pressed ...say, 6, you'd go to 999, be asked for the extension number, then PIN. So...if you called your own extension, got voicemail, then starred out, you'd reach the IVR.
Have not actually used this, but worth a try.