Can callers leave a voicemail when the main away greeting is in use?

No, the main away greeting does not allow customers to leave a voicemail. When it is enabled, callers will hear the away greeting as soon as they call, and will then be disconnected without being prompted to select an extension or leave a voicemail. 

If you'd prefer that customers have the option to access your extensions or leave a voicemail, you can instead create a new main greeting that lets customers know you are away. In this case, you may also want to set up away greetings for each extension as well, which do allow customers to leave a voicemail.