My first Vonage back in 2003 was an ATA186, but the site recommended a Netgear 4 port router. I picked up both pieces through Vonage.
Even Vonage saw the need to provide a router solution (for an extra $40.)

My WRT54GL V1.1 running Tomato Firmware v1.23.1607 has been flawless for the time I've had VOIPo. Before that was a US Robotics modem for a
short bit. Ironically I've never forwarded ports with any service I've used(See my sig), and I've been on Wide Open West Cable, and Comcast for internet.
I always find it amazing that so many people can have so many issues (Be it Vonage, VOIPo, you-name-it ...) I often feel the conditions of success are
affected by:

1) Router
2) Local Network
3) Internet Provider
4) ATA device
5) VoIP provider
in that order.

I have no scientific proof, just observations and gut opinion.