If the message says you have a device in your account already you will need to get a hold of customer service to have them clear it. If you don't want to wait on chat I would suggest Facebook messenger and send them a request that way. It was just as fast for me a few weeks ago than chat.
I had that problem but decided a month ago to buy a phone elsewhere to save some money. I found a phone on eBay that was compatible with the new plan since my current year old phone that I bought here supposedly isn't. I got it cheap and it works, the seller had bought it here earlier this year.
