VoIP server software - which to choose?


New Member
I want to run my own voip server, just for learning purposes. And I'm thinking about software solution.

I think I will have no more than 20 users, maybe up to two concurent calls, so - not much. I have CentOS7 server, with some services running (samba, apache, maybe others) on Core2Quad with 8GB RAM and lot of disk space.
There are many options - asterisk, freepbx, kamailio, others, some hybrid solutions... I need something small and not hungry for resources.

- I will consider virtualization
- not planning outgoing calls; this is a hobby, which shouldn't generate costs :)
- I prefer "easier-to-start" solution
- I expect standards, to let me and others use standard software clients (at the beginning for sure)

Could someone help me with the choice?