Voice quality has climbed back up over the past year in calls to most places covered by the calling plans. Proxy servers have service troubles and/or have maintenance interruptions from time to time, but this is easily overcome by switching servers. That's why they have seven of them.
LAX is mostly fine, but DCA2 was providing better service a couple of months ago.
I haven't needed to call Customer Support in many months, so you'll have to read the postings here to get a better read on that, along with getting back up to speed on the other points you make.
The
SPA-3000 is not a switch; it is an ATA. As such, it cannot make decisions based on availability, time of day, congestion, etc.