I’m going to recommend vanilla Asterisk. We’ve been it running as a central PBX for a municipal government for over a decade now, over 300 extensions at 40 different sites. The /etc/asterisk config files are pretty simple to setup and maintain once you’ve gone through them a couple of times.
Check with local CLECs on SIP trunk service. If nothing else, look at VoIP.ms or Vitelity… Less than $1 per month for a DID and less than a penny per minute of actual usage.