SoftBank Group Corp. is in talks to sell a significant portion of its T-Mobile US Inc. stake to controlling shareholder Deutsche Telekom AG as the Japanese technology conglomerate scrambles to raise funds.

The transaction, if completed, would boost Deutsche Telekom’s nearly-44% stake in T-Mobile above 50%, according to people familiar with the matter. The German company already has voting control of the U.S. mobile-phone giant under a prior agreement with SoftBank, which recently held almost 25% of T-Mobile’s common stock, according to FactSet.