Voicemail greetings can include any information you’d wish to convey, such as special sales, bargains, alternate phone numbers to use, or your company’s normal working hours.
For a VoIP system to work, it needs a means of routing calls between users or to the outside world. In a cloud-based system, this gets handled by a virtual PBX. In the cloud, this routing is managed by your VoIP provider, which is part of why you're paying them. Whatever vendor is supplying that is also running a large PBX operation in a data center somewhere, and slicing off a little of it to dedicate to your organization in exchange for your subscription fees.

Calls to this group would reach Winnie T. Pooh’s phone first before switching to Charlie Chaplin’s phone. When the slider bars overlap in the GIF, this is where the same call would ring both users simultaneously. When the series of calls ends, it can terminate at a voicemail box of your choosing. Because Winnie T. Pooh would be the department lead in this situation, the caller would ultimately reach his voicemail if neither individual in the group answered their phones.
Most VoIP solutions will require stable and consistent internet connectivity at all your office locations where VoIP will be used. At the very least, your business phone system must have access to a business-class internet link, so discuss these needs with your company's internet service provider (ISP). This should be a dedicated link through a dedicated router if you expect your phone calls to sound as if they were coming from a business and not someone's home Skype connection. At a minimum, it's important to have a router that can create virtual LANs (VLANs) and also has the ability to encrypt your voice traffic. These days, you need end-to-end VoIP security for any call.