The war between wireless carriers over customers is heating up, and the battle between ATT and T-Mobile is getting especially fierce. ATT is now offering a $100 bill credit for customers who add an extra phone or data service line, a way of rewarding its current customers while enticing people to switch from Verizon, Sprint, T-Mobile and other carriers. The offer comes on top of ATTs generous promise to pay T-Mobile customers up to $450 for switching, which in turn was a response to T-Mobiles bold opening offer to pay new customers up to $350 to break their contracts with ATT, Sprint, or Verizon. T-Mobile blasted its competitor yesterday with a fake press release complete with fabricated quotes from ATT executives.