Later this year, T-Mobile plans to test a service that will allow its subscribers to switch seamlessly between connections to cellular towers and Wi-Fi hotspots, including those in homes and the more than 7,000 it controls in Starbucks outlets, airports and other locations, according to analysts with knowledge of the plans. The company hopes that moving mobile phone traffic off its network will allow it to offer cheaper service and steal customers from cell competitors and landline phone companies like AT&T. “T-Mobile is interested in the replacement or displacement of landline minutes,” said Mark Bolger, director of marketing for T-Mobile. Wi-Fi calling “is one of the technologies that will help us deliver on that promise.” Major phone manufacturers including Nokia, Samsung and Motorola are offering or plan to introduce phones designed for use on both traditional cell and Wi-Fi networks.