You either buy their router with integrated telephone adapter, or you can get the (usually free after rebate, then rebonkered by shipping costs and activation fees) basic rtp300.
If your netgear router supports QOS (quality of service), you can put the vonage box behind the router with your other stuff. If it doesnt, you'll want to put the vonage box between your router/dsl modem and the existing router. Where it may not work, but if it does, it can prioritize your voice traffic. And also stunt off your data traffic flow enough that you'll be happy to sit around for a week waiting for vonage customer support to call you back and adjust the parameters of the box for better balance.
Or put it behind your router and get occasionally choppy phone calls when using the 'net along with a phone call.