A lot of people are complaining that they can’t receive VOIP-calls when they have a Telenet Homegateway as a modem/router-combo. On fora you can find a couple of fixes, like placing your VOIP-phone in DMZ or enabling UPNP. I found non of these a good solution, because they are not fixing the real problem or tend not working at all. Most VOIP phones like my Gigaset C610 IP I bought from WeePee don’t use UPNP, so enabling that is pointless. Routing all incoming traffic to your VOIP-phone is pointless.
How does it work?
VOIP uses two protocols, SIP and RTP. SIP get’s initiated from your phone so that isn’t an issue. When calling, you transmit RTP packets that contain audio. That isn’t an issue either. The problem is you can’t hear the other side’s voice because the RTP packages they’re sending are received by your Telenet Homegateway. The Homegateway is actually a modem/router combo which applies NAT and it doesn’t know to which device it should forward the RTP packets.
So, if you give your VOIP-phone a fixed IP (it’s just for making things easier) and forward the RTP-ports it’s using in Telenet’s controlpanel all things are working fine.
So what’s the real solution?
Browse to mijn.telenet.be and forward the RTP-ports your VOIP-phone is using to it’s internal ip-address. In my case it’s forwarding port range 5004-5020 for UDP traffic to my VOIP-phone’s internal ip. Reboot the damn thing to make sure the settings are applied and enjoy your calls.