I have installed the TDA IPEXT16 in 6 TDA100/200 systems. In two of these systems the IP phones (both the NT265 & NT136) drop calls after 6-8 minutes. Its not at 10 minutes . It just varies. The regular KXT7633 & 7630 phones have no issues. Only the IP phones. Two separate customers unrelated having the same problem. We are NOT using VPN so Panasonic wont offer me much support. We have the DHCP enabled on the ip phones. We've changed IP cards, upgraded version on the EXT16. Also changed routers and or switch at both ends. I chose not to use VPN because quite frankly it seems more complicated and I am not a data guy.I'm a voice guy learning the data side. I havnt had any problems with 4 out 6 of these ip installs. Any ideas why calls just drop at these two sites? Thanks