Dropping connection in all games with voip connected

My friend is haveing discconectiong issues when in all games except WOW. He will connect but after maybe 10 to 15 min (sometimes more or less) he will start getting a connection issue and drop but still remains in good connection in Ventrilo and xfire. We have tried everything from new cables, routers. We have found tho that if he disconnects the Linksys voip from the network he dosent drop. The topology of his network is this

modem -> linksys voip -> belkin Router -> patch panel -> computers

When we removed the linksys voip and just connected the modem to the router like a standerd setup it works with no drops. Any ideas of how to fix this or settings on the voip that could be changed? the voip was setup by the local telecom.