Not sure what you are looking to accomplish. Asterisk can be all of the above in your list. 250 Phones and 44k calls a month should be easily done by a single server. If you want instructions on installing Asterisk, I’d suggest you look at the Asterisk book which is pretty decent. asteriskdocs.org/
I want to know out do load balancing and try to eliminate single pointe of failliure, since we get about 10 new activation a week, i don’t want to a have a major incident and have all our customer down since most of them are usin VoIP as only telephone service…
I was givin the VoIP department because of my technical skills but i having bearly any experience in opensource pbx witch was customize by 2 precedent programer that did not document what they did.