Nothing showed up in my router logs either. Actually, I downloaded and installed Cisco's ATA debug syslog utility, then enabled the debug options in the ATA to capture all the traffic as per their instructions:

https://supportforums.cisco.com/docs/DOC-9897

Since your ATA is provisioned, I suppose you would have to get Support to temporairly enable the debug options in the ATA since you don't have access to the admin settings. I'm unsure whether the RT31P2 has these debug options anyway because I'm using a SPA2102.

After what happened to me, I got to wondering how many of Voipo's customers who have their ATA's directly connected to their modems (or have all those UDP ports open in their router) are being hit with these attacks and causing mysterious reboots.