When I asked similar questions today I was told, You can add a line, then have the rep. switch the iPhone to your number, you should then be able to remove the data plan and all other unnecessary features making a simple second phone. You will be committed to that plan for two years. Unfortunately if you want to remove that new line you will have to pay an early term fee. I would recommend using an old non smart phone to terminate with, that might lower the early term fee. We need a math person to figure out what the cost would be in the long run.
Don't quote me on any of this.