WASHINGTON—T-Mobile US Inc. is getting advice from Corey Lewandowski, the former campaign manager for President Donald Trump, as part of a lobbying effort to help the telecommunications company secure federal approval for its proposed takeover of Sprint Corp.

T-Mobile said late Thursday in a statement it hired Turnberry Solutions in August. “Corey Lewandowski is now affiliated with that firm and they have offered perspective to T-Mobile on a variety of topics, including the pending transaction,” the company said.

According...