This forum includes tips for maintaining the best audio quality possible with the Ooma System. If your Ooma system is having issues with dropped calls, static audio or echo, look here for assistance.
#71164 by thunderbird
Sun Dec 19, 2010 5:18 am
I had a friend send this document. The document is part of a large corporation’s ISD troubleshooting manual. I hope this document helps in understanding “They can’t hear me, etc.” calling issues. The document has been slightly edited to make it more understandable.

ISD Administrator VoIP Fault Tree One Way Calling Issues

VoIP Issue: After a phone call has initiated, one of the parties can transmit and receive voice traffic, while the other party can transmit voice traffic, but not receive voice traffic.

1. Highest Level Fault: VoIP one way calling issues are almost always related to firewalling.
a. Firewalls are typically contained in computers, routers, servers, gateways, stand along firewalls, and sometimes modems, etc.
b. A firewall may allow traffic in one direction, but may block traffic in the other direction, which is why one of the parties can transmit and receive voice traffic, while the other party can transmit voice traffic, but not receive voice traffic.
c. VoIP uses RTP (Real-time Transport Protocol), which carry the actual data for the voice conversation. The RTP ports are typically negotiated between the VoIP proxy server and the user via either the ATA device or soft phone. These RTP ports will usually be in a range between UDP 10,000 to 20,000. Ports are normally selected at random. If all firewall RTP ports, in the range that is normally used by the provider; aren’t open to two way voice traffic, intermittent one way voice service may occur.
i. Correction: Open ports as required.
2. Second highest Fault: VoIP one way calling issues are related to defective phone(s).
a. Correction: Replace defective phone(s).
3. Third highest Fault: VoIP one way calling issues are related to mis-configured, outdated or defective router.
a. Many older routers don’t provide full UDP 10,000 to 20,000 port range. Sometimes third party firmware is available that adds or extends range.
i. Correction: Reconfigure router to open VoIP required ports, or replace outdated or defective router.
4. Forth highest Fault: VoIP one way calling issues are related to mis-configured, outdated or defective modem.
a. Correction: Reconfigure or replace outdated or defective modem.
5. Fifth highest Fault (very remote): Internet provider ports not open or equipment failure.
a. Correction: Contact Internet provider and report problem. Internet provider’s firewall(s) may be blocking voice traffic.
6. Sixth highest Fault (very remote): VoIP provider Gateway issues.
a. The Gateway is where VoIP is exchanged to/from VoIP to Legacy.
b. Most common issue is VoIP provider gateway server(s), to wholesaler(s), to legacy carrier(s) port mismatch.
i. Correction: Report problem to VoIP provider.
Late Add Note: It was pointed out to me that if the Ooma device is connected behind a switch, the switch could be defective and should be considered as one of the things that should be checked.
Last edited by thunderbird on Mon Dec 20, 2010 6:06 am, edited 1 time in total.
#71167 by southsound
Sun Dec 19, 2010 7:45 am
Great document! Thanks for posting this valuable troubleshooting information.
#71189 by stevepierce
Sun Dec 19, 2010 12:25 pm

Thanks for posting this document.

I agree, CHOP is a very frequent VoIP problem with lots of carriers and almost always a NAT or firewall issue.

The iCHOP problem is happening only on some calls to some carriers. This is why I call it iCHOP, intermittent Cannot Hear Other Party.

Moreover, per your very kind and helpful earlier suggestions, I assigned a fixed IP to the Ooma per your suggestion and put that IP address in the DMZ so there is no port filtering on that address so the port filter limitations should not impact OOMA. I still get iCHOP to certain numbers.

Which is why we think this is a carrier specific exchange point. Ooma confirmed a similar carrier specific routing issue when Ooma customers had iCHOP with Comcast voice users. This was in late Aug 2010.

Please remember, this problem is not happening all the time, but when it happens it is consistent.

There is also a higher failure when using Google Voice Call feature to place a call. Which points to an Ooma Google Voice carrier exchange problem.

Again this doesn't happen all the time which is why it makes it so hard to track this problem down.

I am more then willing to make any configuration change someone would like to suggest on the Comcast Router or the Ooma box. Please remember, for some reason on the new DOCSIS 3.0 Comcast 4-port 1Gb cable modems, a single port rule can't include more than 5,000 ports and there appears to be no way to define two 5,000 port rules.

I am also more than willing to work with anyone at Ooma to reslve this issue. Problem is it Day 11 and I can't get a return call. When I call Ooma, the only suggestions they have is to move the Ooma between the router and Cable modem. Ooma apparently doesn't understand that with Comcast Business the Router and cable modem are a single unit. Then they want me to start messing with QoS, we have, we do, and it makes no difference. So then they say they are escalating the call but no one from Ooma calls back.

So we need a solution from someone at Ooma that wants to help fix this problem.


- Steve
#71297 by thunderbird
Tue Dec 21, 2010 5:00 am
For anyone having this problem:
This is most likely a firewall closed or malfunctioning port problem, somewhere in the circuits from your Ooma device, to the legacy carrier or VoIP Provider, and back to your Ooma device. Have you called customer service? This is just a forum and weather Ooma customer service sees your problems are just hit or miss. Call customer service and report your "I can't hear them" problems.

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