Upgrading iPhone, what about voice?

I tried searching for an answer (really!) but couldn't quite find what I was looking for. 

I wanted to upgrade my old iphone 4 (64GB) to a 6+ (128GB) and, of course, I qualify for an upgrade discount. I like ATT, so I'm just going to take the subsidized price all at once ($500 or so). I still have the unlimited data plan grandfathered in, and it looks like I get to keep it. All good so far. I get to the checkout page and it only lists my data plan for services. Is voice free or something? There's no way to select a voice plan. I don't make voice calls all that often, but I do make them occasionally. Sorry if this is a stupid question! I obviously haven't dealt with upgrading a phone in a long awhile!