T-Mobile US Inc.'s chief financial officer called a $15 billion takeover proposal from French wireless company Iliad SA "inadequate" Wednesday, but hinted it may be open to a higher offer.

Braxton Carter, CFO of the fourth largest U.S. carrier by subscribers, said at an investor conference that Iliad's late-July offer to buy 57% of the company was "very flattering" but "a very inadequate value proposition."

But, he added, "I think rarely people come with their best bid to start."

Iliad has said that it didn't see a need to boost the bid after T-Mobile's longtime suitor, Sprint Corp., yielded to regulatory pressure in abandoning its pursuit.

Last week, Deutsche Telekom AG Chief Executive Timotheus Höttges said he is open to selling T-Mobile at the right price, but has "no offer on the table which increases value more than what we're developing organically." The German carrier is T-Mobile's former parent and still owns 67% of the company.

On Wednesday, Mr. Carter called Iliad founder Xavier Niel a "very impressive entrepreneur."

Write to Thomas Gryta at thomas.gryta@wsj.com

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