More than likely, your carrier's voicemail-handling number. For example, mine go to 805-637-7249 which is T-Mobile's voicemail system.
Which raises an interesting point (honestly, something I just read while googling thie phone number): it seems that the way this works is that if you have an incoming call which you choose not to answer, your phone will forward it to the number cited in settings. According to at least one user, this means that you are effectively charged for the minutes used by your caller when he leaves a voice message. And, in the case I was reading about, since the T-Mobile customer was on a cruise ship at the time, the entire process took place at the international roaming rate of $4.99/minute - even though the recipient was careful not to answer his phone or check voicemail while aboard ship (and thus roaming).