The company has begun a marketing trial in Seattle that links phones to T-Mobil's cellular network outdoors and to Wi-Fi routers indoors, The New York Times reported Tuesday.
The approach seeks to solve the problem of weak reception in buildings.
T-Mobile is offering participants two handsets that cost $49.99 provided customers sign up for a 2-year rate plan for at least $39.99 a month. Subscribers are charged $19.99 a month on top of their regular cellular plan fees.
"For the below-30-age segment, it's a no-brainer," said Roger Entner, a wireless industry analyst at consulting firm Ovum. "This is also a threat for other wireless carriers because it fixes the problem of poor coverage inside homes."