I didn't buy the unlock key. Not necessary for what I tried.
The unlock key is to make sure you don't lose your phone number if you don't use the number for 90 days. It is also necessary to configure an ATA. It may unlock some functions but I haven't seen anything about it.
Like I said: I'm trying it but it just came out, and I have found much info how to customize. But they have a forum: I'll probably take a look soon.
Dell Voice is associated with Fongo (freephoneline.ca). Their product has been free for a long time.
They probably make money from the 2 cents a minute they charge when you call outside the listed cities (more than 2 cents to other countries). Also the fees for the extras. Have you noticed the Dell publicity too?