T-Mobile US Inc. (TMUS) and Sprint Corp. (S) are close to agreeing on initial terms for a merger agreement that could close as soon as the end of October, insiders told Reuters.

Sprint's Japanese parent company SoftBank Group Corp. (SFTBF) would own between 40% and 50% of the combined firm, while T-Mobile parent company Deutsche Telekom AG (DTEGY) would hold a majority stake.

Sources said the deal term sheet is being finalized and both companies will carry out their own due diligence. Though it's possible talks may end unsuccessfully, this deal would merge the third- and fourth-largest U.S. wireless carriers.

T-Mobile stock traded up 0.47% to $63.69 in midday trading on Friday, Sept. 22, while Sprint shares gained 4.55% to $8.40.

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