I managed a set of Asterisk PBXen for about ten years. They're extremely capable. To get full use of it back then, using high-quality hardware SIP phones was a

I managed a set of Asterisk PBXen for about ten years. They're extremely capable. To get full use of it back then, using high-quality hardware SIP phones was a necessity. These days I think that in-browser phones will work very well for most people. Interconnection with telcos went from unsanctioned to orderable to being widely marketed during that time. If you know what you're doing you can save a lot of money. If you would prefer to have someone else do the work, the virtual PBX vendors are pretty good.