This article is making all sorts of illogical conclusions from the stories it's telling. First, how was the company hacked "through their corporate phone system"? They were hacked through social engineering and malware. The phone network was just used for reconnaissance. I'm sure hackers have been mining phone networks for decades. Not that this story isn't interesting, but the conclusion doesn't lead from the tale.…
That depends on the phone system. Asterisk isn't particularly reliable; it actually crashes at 100 concurrent calls. If you're talking about a good CO switch like a 5ESS or DMS-100, they're extremely reliable, well engineered machines. They consistently meet and exceed five nines reliability.
This is a myth. One single Asterisk instance may handle 200-500 calls nowadays, but if you cluster them, you may grow carrier-size networks out of them. There are a number of large scale Asterisk deployments that handle tens of thousands of parallel calls with carrier-grade reliability.