In December, Sprint rolled out a promotion called “Cut Your Bill In Half” meant to attract AT&T and Verizon customers by offering them bill credits. Sprint extended the program to T-Mobile subscribers on Friday. Here’s how it works: Any T-Mobile subscriber can receive credit — at least $200 — for trading in a working and activated T-Mobile smartphone, in addition to a contract buy-out credit worth up to $350 per line. The credit comes in the form of a prepaid Visa card and the deal runs through April 9. These kind of promotions are one big reason that Sprint is once again adding postpaid subcribers.