When talking about sound quality I would recommend using g711 (which uses 64kbps) and a separate internet connection (348/348 line) for VoIP it’s affordable or somehow assign higher priority for VoIP traffic on existing connection. Having people surfing/downloading/uploading using the same connection without prioritizing VoIP traffic, most probably will cause degrading sound quality especially when all four lines will be talking. And I would say ‘while emploee internet access is not affecting VoIP traffic’ instead of ‘while VoIP traffic not affecting employee internet access’
voip-info.org/wiki/index.php … onsumption
Another option is installing Digium card for external lines.
Well, that’s what I think I would do.
In general, VoIP is not very friendly with fax. You can find more about this here -> voip-info.org/wiki-Asterisk+fax