I had this EXACT same issue a month or so ago...
We've got a SB6 128 system running 4. something IVM software. Our box has a 4 port AVC card in it. What we had to do was make sure you've got the newest CHIP for the OCD card and upgrade the IVM software on the voicemail box to 4. latest. (If you have an AVC) I think SS might have a copy for sale.