I use Page Plus Wireless (an MVNO on Verizon's network). I think you can keep the line open by loading the minimum $10 card every 120 days. And they will not delete your phone number for you at least 30 days after it expires, so you could probably stretch the $10 to almost 150 days if you didn't mind the line being inactive for a few weeks.
I think Kitty Wireless will Port your phone number to Page Plus for free (of cheap depending on their current deals), and will automatically load the money as often as you tell them. I am not affiliated with them but currently use them for the same thing and have had no problems.
EDIT: I just noticed that you are coming from AT&T. I don't know, but think you need either a sprint or verizon phone for page plus. But I think there are similar AT&T MVNOs, and you can easily pick up an old verizon flip phone for very little (ebay, craigslist - just make sure that the ESN is clean) and use that to activate the account.