Why did T-Mobile shares jump nearly 8% on Thursday? The wireless giant's earnings report delivered some pleasant surprises.Shares of T-Mobile US (TMUS 6.18%) jumped as much as 7.6% higher on Thursday morning, following the release of strong second-quarter results. As of 1:20 p.m. ET, the stock cooled down slightly to a 6.5% price increase. T-Mobile's popular premium plansT-Mobile added 1.7 million postpaid net customers in the second quarter, led by a doubling of sign-ups for the premium-priced Experience B ...