I recently signed up with Vonage and called them to find out if I could use a Telephone Adapter that I bought on eBay with my existing account. After going to research this issue twice, the 2nd time being asked how I'd get a new MAC address attached to my account, he came back and said:

"Your best bet would be to return your telephone adapter to the store...yup, that's your best bet." < -- is he for real?

The way the system works is that each telephone adapter (as well as every networking device in the world) has its own MAC address (like its unique signature). Vonage ties that MAC address to the phone number they have for you in the system. This is how it works when you take your telephone adapter with you on vacation, plug it in, and you can make and receive calls the same way you did when you were at home.