Masayoshi Son, chief executive officer of SoftBank Corp. , which owns about 80 percent of Sprint, is expected to make a formal bid in June or July, one of the people said.
However, U.S. regulators have expressed concerns about a combination of the third- and fourth-largest wireless carriers in the U.S.
SoftBank and Deutsche Telekom AG currently own a majority stake in T-Mobile US.
In the meantime, T-Mobile US said it added a record number of customers in the first quarter. The company said that 2.4 million subscribers signed up for iits "uncarrier" program in the first quarter -- more than its top three rivals combined, a first for the No. 4 mobile carrier in the United States.
Still, the mobile service provider lost $151 million in the quarter, compared with a profit of $107 million, a year earlier.
The company launched a series of promotions this year, including eliminating contracts and introducing a billing plan for equipment that allows customers to pay for devices in installments.