If they are using a VoIP system, which is fairly common today in large organizations, that means the content of all phone calls would be going over their network and if the hackers had control of the network then theoretically they could have gained access to all calls (as they were being made), not just those that were saved in voice mails.
Hackers may have used VOIP archive. But you can get access to others network.
This hacking is very massive. Hackers have full control. We have not heard about bank and credit cards of DNC, donors and officials.
That would be very unlikely. They would need agent software on nearly every network device and someplace to store all that data until it could be downloaded.
While the VoIP server sets up the call, the voice transmission actually passes via the shortest path between the end points of the call.