Are you using Exchange as your VM?
In any case I suspect what's happening is that a mailbox is set for message delivery. The vm will call your phone and attempt to deliver a message to you. It rings your phone, when you answer it ask for your password. In this case I suspect that the delivery attempt is ending up being rerouted back to voice mail, causing a new voice mail to be delivered. An endless loop unless you can answer that call and enter your password.
Fastest way I know of to stop the problem is to delete the mailbox and recreate it.
Ralph