French mobile operator Iliad has offered to buy T-Mobile US, the fourth-largest U.S. cellular carrier, in a bid that could complicate an offer reportedly in the works at Sprint.Iliad confirmed on Wednesday that it has offered $15 billion in cash for 56.6 percent of T-Mobile. Its bid prices the whole company at $36.2 per share, a premium of 42 percent over T-Mobile's share price on Dec. 12, before speculation about a Sprint bid affected the price, Iliad said.