Latency spikes and inconsistent connectivity

We are having an ongoing issue with our VOIP/Vonage and comcast internet service. We recently upgraded the router to a mutually compatible unit but still have large spikes during ping test which facilitate poor sound quality on phone calls; and occasionally dropped calls. 

Any troubleshooting or advice? Almost unbearable to conduct normal business. 6 phones and 6 pc's in the office, most of which is normal activity. TIVO system for television. Download speed: 18 mbps, Up speed: 3.39 mbps. Phones only require 0.1 mbps per unit.....

Help, please!