Bad call quality usually occurs when sharing voice and data on the same link. We recommend prioritising your network traffic using Quality Of Service; this will allow you to dedicate a portion of your bandwidth to voice to prevent call quality from deteriorating as voice traffic generally is not prioritised over data traffic.
We accept UDP connections on port 5060 and TCP connections on port 5061. Open ports 10000 - 20000 for RTP You will need to have some kind of DSCP marking rule to tag voice packets for the destination of the PBX.