The incompatibility is the ports on ACS and 308 mods fool the VM ports into thinking they are receiving a call. The port answers, but since there is no real call, it never receives mode codes. It times out, and transfers the "call" to the Operator, causing a 1-cycle "ring-beep" at Ext. 10.
We've solved it in the past by reusing a 206 module just for VM, another that upgraded to all 308's needed Avaya to remote into the VM and program a patch.