Sprint Corp has dropped its bid to acquire No. 4 U.S. carrier T-Mobile U.S. Inc after regulatory resistance showed no signs of softening despite months of lobbying, people familiar with the matter told Reuters. The move is a rare setback for Sprints Japanese parent SoftBank Corp , whose billionaire founder Masayoshi Son had seen the acquisition as key to taking on U.S. market leaders ATT Inc and Verizon Communications Inc .