How to make voicemail accessible from an outside line

Here's how to allow your users to call in on your main outside line and access their voicemail boxes.

Install the Misc Destinations module if you have not
already done so: Go to Tools, Module Admin, Check for updates online.
After the page has reloaded, click on Misc Destinations, Install, and
click the Process button.

Create a new Misc. Destination pointing to Dial Voicemail:
Go to Setup, Misc Destinations, Add Misc Destination, give it a
description (such as Voicemail Access), select Dial Voicemail (*98) in the dropdown, click Submit Changes.

Now go to your IVR menu: Setup, IVR. Select you main IVR
menu from the list of IVR's. Click on Increase Options, and when the
page reloads, scroll to the bottom and give the new selection an option
number (what the user dials when in the IVR to get to the Voicemail
menu - note that for added security, you may want to make this a
multi-digit number that cannot easily be guessed by those outside your
organization - just don't let the initial digits be the same as for any
other option). In the Misc Destinations dropdown, select the Misc
Destination you created in the previous step. Click Save, and then the
red bar to commit all your changes.

That's it - when a user dials into your IVR and enters the option
number, they should be prompted to enter their mailbox number (usually
the same as their extension number) and password.

Alternately, if you want to have a DID dedicated to voicemail
access, then (after creating the Misc Destination as shown above)
create an incoming route for that DID: Setup, Incoming Routes, Add
incoming route. Put the DID number in the appropriate field, and under
Set destination, use the Misc Destinations dropdown and select the Misc
Destination you created earlier, then click Submit and then the red
bar. After that, a caller dialing into that DID should be prompted to
enter their mailbox number and password. I don't necessarily recommend
doing this, because if a hacker stumbles across your DID they may try a
"brute force" approach toward finding mailbox number/password
combinations that work, but you can set it up if you and your users are
willing to accept the associated risk.