T-Mobile is banking that handing over cash (well, virtual plastic) will tempt new customers to its unlimited phone plans . If you sign up using its T-Life app, you can get a $200 prepaid Mastercard. Wireless carriers use lots of incentives to bring in new customers, from discounts on phones to introductory rates and price guarantees. Outright offering money is a less common option. Yes, there are details to navigate -- the company isn't just slipping a pair of Benjamins under the table -- but the offer is genuine. Here's how to get it. Who's eligible to get the $200 offer This deal is open to customers opening new T-Mobile accounts, who also have their own phone and are keeping their existing phone number. That number needs to be ported from AT&T, Verizon, Claro, UScellular, Xfinity, Spectrum or Liberty Puerto Rico. How to get it The key to this offer starts by installing T-Mobile's T-Life app; I couldn't find the deal on the company's web site or elsewhere.…