Call your voicemail and listen to the menu. If you don't hear it, go to the additional or extended menu.
If you're in the US, figure 5 seconds per ring (it's by time, not by number of rings) and add 2 seconds, to allow for the fact that what you hear and what the caller hears aren't synchronized. The maximum time is usually 30 seconds.
If you still can't do it that way, call your carrier's customer service number and ask them to reset it for you.