The war between wireless carriers over customers is heating up, and the battle between AT&T and T-Mobile is getting especially fierce. AT&T 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 AT&T’s generous promise to pay T-Mobile customers up to $450 for switching, which in turn was a response to T-Mobile’s bold opening offer to pay new customers up to $350 to break their contracts with AT&T, Sprint, or Verizon. A customer doesn’t need to be on contract to get the credit, but there are some caveats. The offer is only available through the end of March and it looks like the $36 activation fee for new lines still applies.