tethering on At&t

[Socrats]

Quick question. I put froyo 2.2 on my captivate using cognition. We all know froyo allows you to wifi tether. So I've used it a few times in class for kids who are on there touches, but will that run up the bill even if I have at&t 's old unlimited plan? I upgraded from an iphone 3gs to the captivate not to long ago. But my aunt didn't change the unlimited plan or so she says. So am I safe or screwed