Add line for phone promo then cancel other line, will the promo stay on new line?

BuggyBear wrote:

Hi, I was wondering if I get a phone at a promo price for adding a new line what happens if I later need to cancel another line (not the newly added one), would that affect the promo credits for the new phone? Thank you 

As far as I understand it they have a promotion where you get a third voice line at no additional cost. I don't know if that would help but that's what I know.