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Cannot answer incoming VoIP call

jrold
Novice

Cannot answer incoming VoIP call

Router is a Netgear DGN1000 running 1.1.0.41_ww firmware.

Behind the DGN1000, I have several phones which talk SIP to several VoIP providers.

The phones all complete the SIP registrations fine and can make calls fine (outgoing call setup and RTP audio paths are all OK).

Problem is incoming calls. A phone will ring (i.e., the phone receives the SIP INVITE message from the server), but if you answer the call, the SIP 200 OK answer message is never received by the remote SIP server, so the server thinks the call has not been answered and eventually it diverts the caller to voicemail.

Evidently, the DGN1000 is doing something to the phone's 200 OK answer message that is causing it either not to be sent out, or to be sent to the wrong IP address. Either way, it's not received by the SIP server. I have admin access on one of those VoIP servers myself, and can do IP and SIP packet-level debugging to see what is going on there and have confirmed the lack of 200 OK answer at the server end.

I have tried with both SIP ALG enabled and disabled on the DGN1000. No difference.

As a test, I have replaced the Netgear router with a Linksys at which point things work perfectly, which would strongly confirm that the Netgear DGN1000 is causing the problem. Unfortunately, the Linksys is not an ADSL2+ router, so it is not the solution.

I cannot solve this problem using the Netgear's DMZ capability because I need to support several SIP phones and the DMZ capability can only support one IP address.

Is there any known solution for this?

Thanks.
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jrold
Novice

Re: Cannot answer incoming VoIP call

Further investigation reveals that this problem is due to the presence of video codecs in the SIP INVITE and the resulting 200 OK answer response. Removing the video codecs from the SIP INVITE means that the 200 OK answer message also has no video codecs and it is now properly forwarded by the DGN1000 and received by the server at which point the call proceeds normally.

This would appear to indicate a problem with the SIP support on the DGN1000. The presence of video codecs should be allowed and handled properly especially as many users are making video calls using their smartphones these days.
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