bandwidth, packet loss, latency, compression, QoS (quality of service), echo cancellation – notice most of those have to do with your ISP, though how your VOIP phone handles variations in those issues is important too. if your ISP offers a competing VoIP service you may notice theirs is “higher quality” than what you get from a competing company regardless of what handset you use (who needs that pesky net neutrality stuff, we’re your ISP you can trust us!). Also keep in mind, the other person’s connection and hardware (landline or VoIP) plays just as much of a role as yours.