[SR-Users] asking odd mediaproxy,,

I've just setting SIP Express Router and Mediaproxy in 1 PC. Then, I'm setting in NAT Router that only activated for MASQUERADE like this: iptables -A POSTROUTING -s 172.17.2.0/24 -o eth1 -j MASQUERADE NB:I assume in network 10.100.0.0/24 (eth1) as public IP and 172.17.2.0/24as private IP. When I calling with x-lite and activated mediaproxy service, the connection running well with 2 direction (full duplex from 10.x.x.x to 172.x.x.x conversely).But,when I stopping mediaproxy service (for proving that voice packet if pass NAT will connect half duplex or maybe broken),apparently this connection still running well with 2 direction my question,, why this happened? are having trouble with mediaproxy or SIP Express Router? i'm confusing,, thanks very much,, :)