Wow. Thanks for all your replies. It never crossed my mind that it could be an SDS error. The way I described my resilient test wasn't very clear. Here's how I did my test which lead me to believe the problem is with the primary controller.
1) removed the ethernet connection on the primary controller. IP sets re-registered to the resilient controller. These IP sets are extensions 2xxx, 3xxx, and 5xxx
2) on the resilient controller, I programmed/registered a 5330 IP phone with DN 2555.
3) I was able to called 2555 number from another phone on the network, INCLUDING the 2xxx, and 5xxx that just failed over to the resilient controller
4) as soon as I reconnected the ethernet cable on the primary controller, all IP sets re-registered to the primary controller. At this point, I was no longer able to call number 2xxx, and 5xxx numbers.