The ping test is not all that applicable to VoIP--you need to determine true latency and jitter. Anything short of major packet loss is unlikely to cause the call to drop--it's more likely to sound like a crackling CB (this can change depending upon the codec used).

Have you configured the Netgear to open the ports that Vonage calls for? If not, I'd not be suprised if the NAT/PAT timeout on that Netgear is around 15 minutes.