How heavy are those lines being used? Does your company use the 3-way call feature? Does your DVG reboot since the last time you troubleshoot?
I have tried to all of the above suggestions but still no go. We currently have two DVG-3104MS with 7 ATT lines with 150-200 calls a day. The DVGs reboot randomly throughout the day (sometime they lasted for 3 days, sometime 5 minutes). And Calls were dropped around 30 min. during 3-way call.
However, I did talk to D-Link Network engineer, he asked me to wait for Engineers to analyze the packets and he will update me. And it has been almost a month now with no updates.