Unfortunately, once a caller cycles through your auto-attendant options and is introduced to the voicemail option, this automatically end this call after the caller records their voicemail. In order for that caller to get back to the auto-attendant main menu the caller would need to redial that telephone number.
There is one "work around" that might help here. With each individual Voice Mail box, you can go into the Feature Portal, go to the Voicemail Tab, Transfer section. there you can select Activate Transfer, and put in the ext number of your Auto Attendant. That will give incoming callers that are listening to Voice Mail greetings, the ability to press 0, and be routed back to the Auto Attendant. If you do go with this option, you would need to create both a custom No Answer Greeting, and possibly a new Busy greeting, for each Voice Mail box, that notifies the callers that pressing 0 is an option, and letting them know where they will be routed to. I hope this helps.