You can activate any phone you want, as long as it is a Verizon phone. As you stated, you must either pay full retail price (typically $400-$800 for most smartphones) or buy a used phone. You can find some good deals on eBay, Craigslist, or swappa.com, just make sure to get the ESN and make sure it is a clean device before paying for it. Also, if you do decide to go the full price route, purchase and activate at a Verizon corporate store or by calling customer service, so your unlimited data plan is retained. If you're not sure if your local store is a corporate store, you can check on VZW's site. ¡Buena suerte!