I just spent about 2 hours in the local ATT store to attempt to add a "ghost/dummy" line to my unlimited data single line plan with no success. I was passed up the chain on the phone with ATT several times with no results. I was informed that literally as of today (10/25/2013) ATT no longer allows grandfathered single line unlimited data plans add a line and keep the unlimited data, there must already be a second line on the plan or you must give up your data and move to a mobile share plan. In the end I was offered a full upgrade almost 1.5 years early on the iPhone 5S for either $199 or $299 depending on size. They also advised they would waive the upgrade fee and shipping. At this point though I passed on their deal. Instead I am going to buy a used phone from craigslist and seriously consider changing carriers. I can't stand the way they will do anything to make a buck although I fear it is common practice regardless of carrier.