Do you mean V2.1, sounds like you are putting the VM call on hold then attempting to access the same mailbox again, do you get system prompt saying "unable to access due to multiple logins" if so this will drop off after a period not sure bout the length of time, or as you say you can reboot, or just restart VM
Soopa Doopa Intergalactic IP Office installation mechanic and configuration corruptor from way back.