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PostPosted: Wed Aug 26, 2009 4:33 pm    Post subject: Dial tone ok, incoming calls ok, no outgoing calls Reply with quote Back to top


I am having problems with my Vonage service. Currently I can receive calls, and I have a dial tone when I pickup the phone. If I try to dial an outside number however I will hear the dialing and see it dial the correct number on the VPortal but after that silence and no call is made.

Here the information to the setup:
Location: Florida
ISP: Comcast (cable)
Upload/download: not sure, but high enough (>2mbps)
Modem make and model: ARRIS DOCSIS TM602G/CT
Vonage adapter: VDV21-VD
Network: Modem -->Linux router (Vyatta 5.0.2) --> Hub -->Vonage Adapter

I could not complete the Voip test, as I get Java error messages on Firefox and Iexplorer 8.0.

The vyatta router is configured for NAT and has firewall rules active which drops all packets except traffic to/from established connections and some UDP traffic required for VPN access.

I can not install the Vonage adapter in front of the vyatta router as then VPN access to the network would no longer work.

I found a Vonage support document stating the required ports for Vonage to work:

TFTP: 21, 69, 2400 UDP
NTP: 123 UDP
SIP: 5061 UDP
(These are not blocked by the firewall, as it allows response to established connections originating from devices behind the router)

Incoming & Outgoing (for RTP)
10000-20000 UDP

Thinking the 10000-20000 UDP incoming might be the problem I followed the instructions and forwarded these ports to the Vonage adapter which has a fixed IP. This however did not resolve the problem. I also changed the firewall rules to not block UDP traffic for these ports, this did not work either.

When i disable the firewall rules, leaving only NAT and port forwarding enabled, I can make calls though randomly it still fails with the same symptoms described above. Leaving the firewall disabled is not acceptable though as it would leave the network wide open.

Any ideas if I am missing any ports or port ranges or what issue could be causing the problem? The weird thing is that it is outgoing calls that are not working. These calls originate from inside the network and should therefor not be blocked by the firewall (though UDP is connectionless, might this be the issue?).

Any hints or advice are greatly appreaciated.

Thank you in advance,
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PostPosted: Wed Aug 26, 2009 6:05 pm    Post subject: Reply with quote Back to top

I am not familiar with the router you are using but I would suspect that it has some form of stateful packet inspection and it thinks that the RTP audio stream is an attack of some type and something it should block. Is there a log that keeps track of what it blocks?
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Posts: 656
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PostPosted: Wed Aug 26, 2009 10:37 pm    Post subject: Reply with quote Back to top

You would need to determine if this is a non audio issue or a call completion issue. When you place a phone call does the number you call ring at the far end at all? If not, this is not an audio issue. If this is the case there is one thing that comes to mind.

If your Authenticated Invite does not make it to the Vonage proxy, your call will not be processed and far end will not ring. A common cause of this would be an MTU size on your router being set too low. A UDP SIP fragment will not reassemble correctly so you cannot fragment these packets. The Authenticated Invite is the largest of the SIP packets due to containing all the standard headers, plus the SDP information, and your authentication digest.

Make sure that your MTU is 1500 bytes. You can run a ping test to any web site and start with 1500 byte size packets and then start to work your way down to see what your limit is.

Last edited by VonTechMgr on Thu Aug 27, 2009 10:29 am; edited 1 time in total
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PostPosted: Thu Aug 27, 2009 10:21 am    Post subject: Reply with quote Back to top

The problem was indeed that the far end did not ring, so it was not an audio issue.

Inspecting the MTU size, it turned out the vyatta router defaulted the ethernet interfaces to MTU 576.

After increasing it to 1500 the problem was resloved. This probably also explains some of the intermittent problems even when the firewall was off.

Thank you again for the prompt and great help Very Happy

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PostPosted: Thu May 03, 2012 9:11 pm    Post subject: Reply with quote Back to top

Thank you! This was my exact problem. Outgoing calls would fail, incoming calls were OK. Two different techs helped me prove that my firewall seemed to be working, but neither clued in to the possibility of an MTU issue. iptraf was occasionally reporting UDP fragments, and google brought me to this thread.

Turns out that the outside interface on my shorewall firewall was set to an MTU of 576. As soon as I reset it to 1500, everything was great.

Thanks for posting this information! I wish I had found it two days ago!
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