Listen voice messages from one asterisk box to another

I have RealTime asterisk with 3 servers. In database I hold sippears only and voicemail boxes.
Server A and B are for calls and registers and Server C is dundi.
Currently everything work fine… I can call from Server A to Server B. The problem is when I leave message to number who is busy and registered on Server B… then this number disconnect and register on Server A -> he can’t listen the messages because it is stored on Server B…

How can I make any user to be able to listen his messages no matter on which server are?