Iliad SA, the French mobile-phone carrier founded by billionaire Xavier Niel, offered $15 billion in cash for a controlling stake in T-Mobile US Inc., rivaling a proposal from Sprint Corp.

T-Mobile parent company Deutsche Telekom AG considers the $33-a-share offer for a 56.6 percent stake as not competitive and inferior to a separate bid planned by Sprint, said a person familiar with the matter. Masayoshi Son, the billionaire whose SoftBank Corp. owns Sprint, is said to be working on an offer of about $40 a share for T-Mobile.

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