I got a solution for this, so wanted to post it for the archives...

Many of the PAP2T's being sent out came from a European source, and have European call settings loaded. To get a Uniden (and possibly other phones) to ring, here are the settings needed:

Ring Waveform: Sinusoid (Trapezoid will work, but it has a "stuttery" sound)
Ring Frequency: 20
Ring Voltage: 85
CWT Frequency: 440@-10
Synchronized Ring: yes

The critical value here seems to be the Ring Frequency. European phones use 25, US phones use 20.

Support now has this issue in their knowledge-base, so they should be pushing it out for future Linksys ATAs.