T-Mobile falls as KeyBanc downgrades to 'underweight'

** Shares of wireless carrier T-Mobile TMUS fall 1.8% to $231.8 premarket

** Brokerage KeyBanc Capital Markets downgrades TMUS to "underweight" from "sector weight"

** "We think TMUS's consumer value prop has deteriorated due to recent pricing actions, which suggest TMUS shouldn't take outsized share of the industry net adds," brokerage says

** 19 of 30 brokerages rate the stock "buy" or higher, 10 "hold" and 1 "sell"; their median PT is $271

** Up to last close, stock had risen 6.9% YTD