Masayoshi Son, head of SoftBank Corp. and Sprint Corp., won't be able to ride the regulatory coattails of pending U.S. telecommunications deals if he decides to buy T-Mobile US Inc.

Regulators are preparing to vet proposed acquisitions worth more than $45 billion each from Comcast Corp. and AT&T Inc. Those transactions, involving pay-television services, pose different issues for competition and the public interest than the combination of wireless-service providers Sprint and T-Mobile.

Of the pending deals, a Sprint purchase of T-Mobile is "going to be the hardest one to get through," said Jeff Blumenfeld, an attorney at Lowenstein Sandler LLP. "You have direct competitive overlap in an already concentrated industry."