Will this work? Adding iPhone line to premier

[asr10 user]

Sorry, can someone help me with this. The ATT premier website is a bit confusing to me. I am currently on an individual plan with unlimited data. I also pay for my grandmothers prepaid phone and am going to buy an iPhone 4.

So I have this idea of adding a line for my grandma and buying the iphone at discount for her line, then using her dummy phone for her line, canceling data on her line and swapping the iphone over to my line with unlimited data. This should work correct? And I would be paying the same price im paying now plus the $10 for a new line correct?

The only hassle would be getting new sims for both the iphone on my line, and a regular sim for her phone.

And to actually add a line to my individual premier account, do I add it as an individual line then call to have it become a family plan? Or do I select family plan and leave all the options for the first line empty?

Am I missing something, or should this work?