How to implement sip communication for 20 people?

Question

SIP

So. We are switching from analog PBX to sip-telephony. No money. 20 jobs. Virtual machines are spinning on servers (in theory, I can forward the board inside the virtual machine. Probably). Unless, of course, this option is considered optimal. 2 external analog lines. There is a third, but it can already be realized from a good life. I would like to take something small and autonomous (ats). The switch has poe. On the ground, you need to put something with tubes (so that people can walk around the office). Plus ... but this is in theory ... redirect calls to soft clients on smartphones and some computers. Of the functions ... this is the forwarding of external calls to the sip provider and 6 simultaneous calls. Advise ats cheaper. but high-quality + sip-phones with separate handsets (preferably with 2 lan ports, preferably with gigabit ones, but not critical).
ps Do not offer automatic telephone exchanges above 10 rubles! There is no money wow. Unless there are some serious excuses.
The office is tied with a local network, a waffle and some people sit through vpn.

Answer 1

asterisk + old sistemnik. And e-bay with old tsiska 7940