3rd Party VoIP Outgoing Call Issues - NVG589 Firmware Ugprade 9.1.0h12d22?

So I got a brand new Motorola NVG589 about a month ago to replace my 2Wire modem for my UVerse Internet service. I have VoIP with JoiPhone running through a Linksys RTP300. The RTP300 is directly connected to the NVG589. JoiPhone worked fine through a DMZ with the 2Wire.

I was initially able to get JoiPhone to work fine with the NVG589 by simply opening all of the ports with a custom service under Firewall -> NAT/Gaming. This was on 7/26/14.

I had no JoiPhone problems until 8/10/14. Then I started having one-way voice issues. I have a dialtone, I can call out, and the calls go through (I get missed message alerts on my cell), but I cannot hear anyone on the other end. So far I am able to hear when someone who calls me; the issue seems to be outgoing calls only.

I assigned the RTP300 a fixed IP address, and I placed it in IP Passthrough - DHCP-Fixed (I tried the Manual option, too). Same problem. I called JoiPhone and they did the same thing. They also tried to open the JoiPhone specific TCP/UDP ports in NAT/Gaming. Same problem. There were multiple power cylces of both the NVG589 and RTP300. JoiPhone recommended that the NVG589 be Bridged and/or that SIP ALG be disabled. I checked with AT&T and neither of these options are available for the NVG589.

I called AT&T and found out that I was pushed firmware version 9.1.0h12d22 on 8/6/14. It's certainly possible that this started the problems because it's unlikely that we used the phone between 8/6 and 8/10. Does anyone know what could have changed in the firmware update? Is there any way to roll it back, or is the prior one out there somewhere for troubleshooting?

I found this quote in another post: "The U-Verse service requires the customer to use a router that was designed specifically for U-Verse Internet, TV, and phone service.  This router has a default setting called SIP-ALG that cannot be disabled and the router cannot be replaced while using U-Verse Internet.  SIP-ALG is a router function that will cause VoIP traffic to be rewritten, which can cause problems such as one way audio, dropped calls, and inbound and outbound call failure.  If you are familiar with making changes to your voice over IP phone/device, you will want to modify the source port of the phone to a port other than 5060 and reboot the U-Verse router."

I don't have access to those settings in the RTP300, but I'm checking with JoiPhone to see if there's anything I can do. It still doesn't make sense that the phone worked for 3 weeks on source port 5060 and then started having issues unless there was a change in the firmware upgrade (or my RTP300 just flaked out--but then I wouldn't expect it to be working halfway).

Any other ideas to try?