Grandfathered Simple Choice

I have a really old plan that is only voice. To get data, we added 3GB of high speed data per line for $5, which got upgraded to unlimited data last year as part of a T-Mobile special upgrade for all existing plans that had caps. My problem is that I want to add a new line of voice service and data, but now T-Mo wants to charge me $20 for 3GB of data. T-Mo reps said that we can't get the $5 deal on new lines any more. But I'm wondering is there any way to ask them to give us the same $5 data deal that we currently have on all our original lines? If so, who would I talk to? ¡Gracias!