De link :
"One common cause of one-way or no-way audio is when Network Address Translation (NAT), Port Address Translation (PAT), or firewalls exist between two endpoints. The SCCP protocol embeds IP addresses in the IP packet's payload to signal which IP address to send RTP packets to. If the device performing NAT or PAT is unaware of this fact, the embedded IP addresses are not translated. Therefore, one-way or no-way audio results."
Use o link no seu gateway. Ele escuta na porta 2000 e redireciona as chamadas do seu telefone IP para o gerenciador de chamadas. Use:
iptables -t nat -A PREROUTING -i $LAN_IFACE -p tcp --dport 2000 -j REDIRECT --to-ports 2000
para redirecionar o tráfego da LAN para o proxy magro. Funciona.