Yup I agree with the other guys your router is stopping data coming back through it.
voice is using tcp/udp ports 1024
tcp ports 1503,1720,1731,522,389
well video also but open your web browser of the router and find something that mention virtual server or port forwarding local port and the other port mentioned should be the same eg: port 1720 maps to 1720 port ends @ 1720 or someting like that !