It only lets you make facetime calls when you're on wireless as standard, not 3g, so it shouldn't cost you and the data usage isn't going to be huge, so unless you have a small download limit then it's unlikely to be a problem.
Thank you .got 10 gig is it use about 5 a month so it would be quite severe on it then .Would bt warn me I was using up my allocated .Is it the same usage to recieve a call as to make a call on face time