I have a Samsung Dart T-499 and i have the $30 monthly prepaid plan that includes 100 minutes, unlimited text and unlimited internet (with the first 5 gigs at 4g). I was looking at the tethering service for $15 dollars a month but found out that my phone already does it, i just need to turn it on. What's the difference between paying the extra $15 dollars a month and just using the built in feature? Does the extra $15 a month still use up my internet and make me reach the 5 gigs faster? Or is it unlimited use at 4g speed?