He did say VPU, and that makes sense at v8.2.

As previously mentioned, the most common cause of no audio is lack of connection on the audio cable. Every Axxess with voice mail has an audio cable. The EM and VPU have a round cable with a 90 degree connector on the CPC. The EMVC has a ribbon cable between the VM card and CPC.

I suspect that the serial cable referenced is actually the audio cable, and integration is via a LAN connection. I would start by reseating the audio cable at both ends.