Whilst 3CX are good, it costs for a lot of things.
Trixbox CE (www.trixbox.com) will do more than you ever dreamed possible for completely free. For stability, stick with Asterisk 1.4 as my personal opinion is that Asterisk 1.6 isn't quite production software.
/me dons fireproof coat for that statement!
As such, Trixbox 2.6.2.3 would be ideal and is easy to setup and configure. Just make sure that you reduce your attack surface area by only allowing permitted IPs SIP access through your firewall and also, whilst not as much use these days, installing fail2ban is a good idea.
Adam
Sky Max LLU