Asterisk with Dlink DVG-6008S -Drops call randomly

We have newly installed Asterisk call server with DVG-6008S and the calls get dropped randomly. There is no fixed time or duration of calls when it drops.

We are using Cisco-SPA-502G phones around 24 of them. The firmware on the Dlink is upto date and event he asterisk is updated.

Is there any specific settings need to made in dlink or server.

Help needed.

sip set debug on

Then try and correlate the output with a lost call.