It seems like software PBX vendors are very "coy" about saying what hardware is required for what call volume. A lot of equivacating, etc. What most people want is just an idea: What are people doing out there? In this blog post I'll give what some brave vendors are giving as examples.
Brekeke PBX (Windows/Linux)
Brekeke has a Windows based PBX product and they have a nice chart in their electronic user manual at http://wiki.brekeke.com/wiki/BPBX_History . I've been hearing good things about these folks and just noticed this very nice chart as I was perusing their product. Below is a screenshot of the page:
Yeastar BizPBX (Windows/Linux)
At this page http://www.yeastar.com/Products/BizPBX.asp Yeastar say that:
Intel Core Duo & 2G RAM for 64 Simultaneous calls & 4 Skype trunks; Windows XP Professional SP2 or Windows 2003 SP2.This one is a bit unusual as they throw in 4 Skype trunks. But one thing it shows: Skype trunks are "expensive" performance wise!
snom ONE Windows Phone System (Linux/Mac/Windows)
snom ONE is a B2BUA system so all traffic flows through the PBX. snom suggest 150 extensions as a good system size for todays servers using snom ONE. But on their forum they note that many more extensions can be registered (1500 extension system in production) as this takes very little CPU--the key is simultaneous calls. In this post it is noted the 250 extensions and 50 simultaneous calls should not be a problem on todays servers.
3CX Phone System
In a blog post in 2010, 3CX did some benchmarking on a Intel Atom processor and concluded that they will handle roughly 20-30 calls well. You can read the whole article here:
If you have more SIP PBX server sizing resources, I'd be glad to hear from you!