When you say lines, do you mean extensions with separate numbers? If so, you can put the same email on the notifications for each extension and number. This way, you can be notified at your email of voice-mails from each of your "lines" and not have to log-in to each extension separately. You would be able to tell which number the voicemail was left at.
This would also enable you to check voicemail via mobile if you have your email on your phone.
There is also an option to get text notifications if data use might be an issue, but this would only inform you that you have a voicemail.