T-Mobile is doing its best to entice iPhone users to switch to its network, offering a substantial discount to people trading in their existing iPhone 4s and 4Ss. Timed to coincide with T-Mobile’s Friday iPhone 5 launch, the promotion will let qualified customers swap their past iPhones for new iPhone 5s, avoiding the ordinary $99 upfront payment, and getting a credit of up to $120 toward their monthly bills. Split over the 24 months it takes to pay the device off, that $120 would knock monthly payments for a 16GB iPhone 5 back from $20 to $15. The promotion lasts until Father’s Day (June 16th), so if you're thinking of making the switch to T-Mobile, you still have plenty of time to see what you can get for your old phone on Craigslist.