When you had CAllVantage, did you have the same setup?

Modem --> ATA --> Personal Router --> PCs/other network devices

Also, what's your upstream bandwidth typically (with no call), 500k? (Perhaps you mentioned it in your post, but I missed it.)

I'm not familiar with the RT31P2, but if you have a VoIP ATA as the first device in line from the modem, it can seriously throttle your traffic, if that's what it's programmed to do. In a way it's funny you mentioned ATTCV, because their DLink DVG1120M was notorious (especially early in the service when that was the only ATA they offered) for throttling people's connections...

Here's what I recommend that you consider, but it does depend if your router has QoS capabilities. (If not, you may not want to switch your network around.) Put your router directly to your modem, then plug your ATA into one of the router's LAN ports. Set your ATA to have highest-priority QoS.

Once in that setup, place a test call to a reliable destination (e.g. a friend who will tell you if your phone connection is good or bad), and start a 2-way speed test (e.g. at dslreports.com). If the upload speed is improved relative to what you're seeing on the phone now, start uploading a folder full of 5MP-or-greater pictures to Snapfish, and ask your friend how you sound. If you like the answer, keep this setup.