There's no charge and, I'm told, no contract change.
The A List is a set of numbers that can be dialed without a "per minute" charge. I think they're only available on higher end plans. On our family plan we get 10 numbers.
Since in-network mobile calls are already free you don't want to add those. We added our home number (Qwest), my office phone, and several Google Voice numbers (let's see if AT&T allows those!). If this works I won't even spend minutes when I use GV to call Canada.
Numbers must be within the US, there are a few exclusions but I don't recall them all and I can't retrieve the list. Google Voice wasn't mentioned.
Numbers currently take a day or so to be activated.
Update 9/28: All of my numbers were accepted, including our Google Voice numbers. I think people eligible for this feature also have free calls to AT&T mobile phone subscribers; it this is correct you would want to avoid using up an A List slot this way. I think the system will allow you to add an AT&T mobile phone number to the "A List".
Update 9/28b: See comments. " ... you need to be on a 1400+min/mo family plan or a 900+min/mo individual plan for this to be offered." In other words, this makes for good marketing, but no impact on AT&T's revenues. Admirably diabolic.
It might help with 3-4 phone families, though nowadays young-uns don't talk much anyway -- and AT&T isn't reducing their texting fee!
We are really moving to a flat rate for unlimited voice, and a crazy AT&T revenue stream from texting- a cost structure completely disconnected from bandwidth and infrastructure load. It's a weird wired world these days.