[dubwubz]

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.

[MichaelGG]

Not that I have any love for Asterisk, but modern versions can handle that load. As can FreeSWITCH despite its... interesting... threading model, as can OpenSIPS. On top of that, who cares what a single process can run? It's better to run one per core anyways. I had several servers, each running several FS instances and scaling wasn't really an issue.

I even ran a 5 nines 9-1-1 service off Asterisk, as embarrassing as that is. We missed one call in 18 months during a hard maintenance cutover, but handled it manually, but of course it was only one part and relied on the rest of the PSTN.

With a SIP proxy in an HA setup, almost surely the biggest downtime is going to be operator error or IP connectivity issues. Or someone changing their signalling subtly, but OTOH 5ESS and DMS aren't totally identical in behavior either.

(Though fair enough, the "software" guys haven't shown the same uptimes, overall, as the old style switching, true.)

[kimi]

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.