There are two ways you can go. The simplest is to put the VOIPo adapter between the cable modem and the router. This works for most people, but didn't for me. The alternative is to connect the adapter to an available Ethernet port on the router. This may work with no additional setup, but some people need to set up "port forwarding" on the router (and reserve an IP address for the adapter.) This what I do.

I will comment that if you go the first way, you will probably need to have your router release its IP reservation before disconnecting it and then connecting in the adapter, as then your ISP will see a different MAC address and it may refuse to allow it to connect.