When it fails a new one is chosen, but while UDP NAT sessions are alive on the new gateway, it is used regardless the status of the preferred one.
This sounds pretty logical in regard of our situation. And indeed, this issue affected only our VoIP services.
By the way, i didn’t find how to configure/disable SIP user agents you talk about. Where can i do that?
So i guess to solve this issue, you have to set UDP NAT session timeout to very short time… ?