Sprint Nextel has seen its biggest fall in share price for three months, reports Reuters, after the US number-three operator was downgraded to "underperform" by an influential investment research firm. Sanford C. Bernstein predicted Sprint will fail to sell enough iPhones to cover its high cost commitment to Apple for the right to carry the device. The company committed to a US$15.5 billion four-year agreement with Apple to sell 25 million to 30 million iPhones. Costly network upgrades, a “stupendous debt burden” and a “hobbled 4G offering” were also cited as challenges facing Sprint in Craig Moffett’s research note.