Please check your router's configuration

Is your VoIP box hooked up to a router?  The router's built-in firewall can slow down the traffic to/from your VoIP box.  You can try to put the VoIP box outside of the router's firewall, like DMZ or DMZ+.

I noticed significant quality improvement after I did that with my ooma telo.  I have a Cisco/Linksys router.