I am running a Samsung Office server100 Phone system in my office with 4 IP phones for staff to work at home. I have set the routers quality of service to give 25% of the bandwidth to the phones systems IP address. ( The phones work fine to out side lines or any phones in the office but when they connect to each other they clash and reboot I suspect that i need to give Quality of service to each Phone but how? would I use the Wan IP of the staff's home routers or something else (Router is DrayTek Vigor 2800 with VPN to our London office)

Thanks for any help.