Fair question Dignan, and neither had I.
I've use DD-WRT in the past, but it is now near impossible to find a straight answer on their website, the router database conflicts with the wiki, which conflicts with the threads. Most recently I've been using vanialla version of Tomato and TomatoUSB, but they don't support the Atheros chipset. So I went hunting. I tried to use OpenWrt but the lack of a GUI and drivers in the base install made for too steep of a learning curve. Hence I ended up at Gargoyle, which is based on OpenWrt.

Gargoyle does have some interesting features to it, though I didn't miss not having them when I was using TomatoUSB. I'm also finding an issue when I enable N wireless on the WNDR3700v1 - speedtest.net tests return about a fifth of the performance, and occasionally fail the upload test. Not that I need N...

If I can't resolve the DTMF tones issue, which is likely just a setting on the ATA or at my provider that needs to be tweaked, I'll flash the router back to OEM and Freecycle it. If someone here wants it, it is their for the cost of shipping.