AT&T U-verse → VOIP phones not working with router behind NVG510

This is an update of sorts from my post last week, “Ditch the NVG510…”

To summarize, I have a small business network with a Netgear SRXN3205 router and a Netgear Smartswitch sitting behind the newly installed NVG510. I have 4 computers, 4 printers and 3 hosted VOIP phones on the network. Everything ran well on the AT&T DSL modem in bridge mode, except I could not get adequate upload speed for the IP phones, even with load balancing on the Netgear router. So I decided to move to U-verse ASDSL2+ a couple of weeks ago and was introduced to the NVG510.

I have had no luck thus far in getting everything working if I use the Netgear router behind the NVG510. I first tried to configure it using pass-through configuration described on Ron Berman’s site, but I could not get the phones to work. As an experiment, I bypassed the Netgear router and used the NVG510 as the network router, and the phones work. However, that forfeits all of the advanced features of my Netgear router, including the advanced firewall, the VPN and the QOS features.

Last Thursday, a very good technician from AT&T spent over three hours in my office trying to get everything configured through the Netgear router. Instead of using the pass-through configuration, he assigned a static IP to the Netgear router, and everything worked but the phones. He could not figure out how to get my VoIP phones to work, and suggested that I contact the hosted IP phone provider for assistance.

On Friday, I spent over two hours with my hosted IP phone provider support department, and a very qualified technician there remoted into my system. He was very knowledgeable and was able to quickly telnet into the NVG510 and enable the sip feature, thinking that might be what was blocking the phones. After that did not cure the problem, he removed the fixed IP configuration of the router, and could get the phones working if plugged directly into the AT&T modem, but not if plugged into the Netgear router. After two hours of effort, he concluded that the AT&T technician must have done something to my router that was preventing the phones from connecting with the Internet. I do not believe that to be the case, but I am certainly no expert when it comes to routers or networking. Again, everything worked perfectly before I switched to U-verse.

My questions concern whether anyone else has had luck configuring an enterprise grade router behind the NVG510, and successfully using VoIP phones with it. I also wonder if anyone has any suggestions as to what might be the cause of my router blocking the phones from connecting to the Internet through the NVG510. Any suggestions on proper configuration of the AT&T modem beyond what I have found through Internet searches would be greatly appreciated. It seems that AT&T should provide more guidance on these issues, since a bridge mode is not available for this modem.

Would it help to assign the other static IPs to the phones? One other less-desired option might be to keep the static IP on the router and run everything through it except the phones, which I plug directly into LAN ports of the NVG510, but I am not sure that this is possible.

These look like they might help:
»getsocial.att.com/docs/DOC-1157
»[solved] NVG510 DMZ/Passthrough/Bridge to another router?

Also:
»Rooting the NVG510 for true bridge mode

Perhaps Passthrough has sufficient limitations versus true bridge-mode, but may also prove a waste of effort to try.

Likely you've tried and failed to find options or they had no effect (on NVG510 and/or Netgear) to disable SIP-ALG (an often broken NAT feature that sometimes fails VOIP)? Do you know if failure is in initial call-setup, versus the audio/media traffic?

Do smartphone VOIP apps work on your LAN? If you find some that do, some that don't, that might help other forums readers spot your problem. Some have many options, if you have time to slog thru, despite no guarantee of improvement. Media5-fone (on iOS at least) has a viewable traces log that may also yield clues (to someone more knowledgeable) about why a connection fails.

Nothing worked at all after hours of efforts, other than running the phones directly from the NVG and everything else from the Netgear. But that forfeited the QOS features of the router, so I took the plunge and rooted the NVG510 and put it in true bridge mode, and now everything works great. Problem solved! Thanks for the link, brookeKridge!