How can we access our voicemail box from outside the company/office to check voicemail and change voicemail greetings?

You can configure IVR to do that. Navigate to IVR and set 'Remote access' to 'Voicemail'.

When you enter number that you configured in IVR, you will be asked for an extension number and PIN.

After that you will be able to check your voicemail and change voicemail settings.