* Iliad bids $15 billion cash for 56.6 pct at $33 per share
* Deal to be financed with cash and equity; banks lined up
* Sets up potential bidding war with Sprint
* Iliad faces fewer antitrust issues than Sprint (Adds challenges facing Iliad bid, background on Niel)
PARIS/NEW YORK, July 31 (Reuters) - French telecommunications company Iliad SA has made a surprise offer for T-Mobile US Inc, setting up a potential bidding war with Sprint Corp, the U.S. mobile carrier now controlled by Japan's Softbank Corp.
Iliad, which has shaken up the French mobile and broadband market in the past decade with its cheap, pared-down subscriber plans, bid $15 billion in cash for 56.6 percent of T-Mobile US at $33 per share, it said in a statement on Thursday.
The Paris-based company said its offer for the fourth-largest U.S. carrier values all of T-Mobile at $36.20 per share, a premium of 42 percent over the pre-announcement share price.
That is less than the roughly $40 per share Sprint agreed to pay under the broad terms of an agreement worked out with Deutsche Telekom AG, T-Mobile's majority owner. The terms of that proposal, which followed months of talks and which was reported by Reuters in early June, would value T-Mobile at nearly $32 billion.
Deutsche Telekom and Sprint declined to comment, and a representative for Softbank could not be reached.
Despite Iliad's lower offer, a person close to the French company said founder Xavier Niel believes he has a strong card to play because his bid would not face the antitrust scrutiny that confronts Sprint in trying to merge the third and fourth-biggest U.S. mobile operators.
"SoftBank has been told in many very clear coded words that the Department of Justice and the FCC would probably not approve the acquisition," said Reed Hundt, a former chairman of the U.S. Federal Communications Commission. "There's no question to me that the FCC would say 'bienvenue'" to the proposed Iliad deal.
The FCC and Department of Justice expressed a desire earlier this year to have at least two more network operators competing against AT&T and Verizon.
The T-Mobile offer is Niel's most audacious attempt at extending his reach beyond France, Monaco and Israel, where he owns part of operator Golan Telecom. Still, his bid to enter the United States mobile market is a long shot, some investors and analysts say.
The French company specializes in broadband and lacks experience in mobile, T-Mobile's main business, having launched its mobile service only in 2012. It is also unfamiliar with the demands of competing in the United States, with its massive coverage needs and deep-pocketed competition from AT&T Inc and Verizon Communications Inc, the market leaders.
Iliad expects $10 billion in savings from the deal, although it is unclear where they would come from because the company has no U.S. operations.
The Iliad offer could falter on price alone, said Joseph Mastrogiovanni and Michael Baresich, analysts at CreditSuisse.
"We are sceptical that T-Mobile and its shareholders, including Deutsche Telekom, will find this bid attractive," they wrote in a research note. "However, it could put pressure on Sprint to move sooner rather later.
Few doubt the scale of Neil's ambitions. The entrepreneur, an unknown outsider in France when he started out, has joined the elite, lunching with ministers, starting a tech school, and holding part ownership of the influential Le Monde newspaper.
He earned his first fortune from an adult chat and dating service on the Minitel, a rudimentary computer network that pre-dated the Internet in France. He then surfed on a wave of market liberalisation in telecoms to create Iliad.
In many ways Niel is similar to Masayoshi Son, the head of Softbank and his rival for T-Mobile US. Both have operated their companies as challengers who cut prices and take on larger rivals with bigger resources.
Niel sees the U.S. market as ripe for the kind of challenge Iliad mounted in France, where its entry into the mobile market in 2012 sent prices down 30 percent and hurt the profits of bigger rivals Orange SA and SFR, as well as Bouygues SA. He ranks 133rd on Forbes' list of billionaires, with a net worth of $9.5 billion.
Son, who is also Sprint's chairman, has pledged to start a price war in the United States, and he has said industry consolidation would allow Sprint to compete more effectively against Verizon and AT&T. He owns 19.3 percent of Softbank and is 46th on the Forbes list, with a net worth of $18.4 billion.
T-Mobile would appear well-suited for the role of challenger championed by Niel and Son. Last year, it turned around years of subscriber losses using a strategy that eliminated contracts, restructured plans and set off a race to slash prices across the industry.
Earlier on Thursday, T-Mobile posted a net profit after a year of losses, and reported the industry's largest post-paid phone subscriber additions of the quarter.
T-Mobile Chief Executive Officer John Legere, known for his outspoken and sometimes abrasive style, has come to define T-Mobile's new audacity, epitomized by his frontal attacks on competitors, offering to pay early termination fees for customers who defect from rivals, for example.
"We know this is a scale industry. Scale brings advantage," T-Mobile Chief Financial Officer Braxton Carter told Reuters earlier on Thursday.
"What we've seen so far is a glimpse of what real competition in this industry looks like. If we could turbo-charge it, it could be an incredible opportunity to bring more competition to the market."
BIG BITE FOR ILIAD
Iliad said it would finance its offer, which was earlier reported by the Wall Street Journal, through a mix of equity and debt, and that it already had the backing of unnamed international banks.
Nevertheless, the deal would be a big bite for Iliad. Its market capitalization of just above $16 billion compared with about $25 billion for T-Mobile US.
T-Mobile owner Deustsche Telekom will now have the benefit of two bidders.
A person at Deutsche Telekom familiar with the talks also said a deal with Iliad had a certain appeal because of the lower risk of being blocked by U.S. regulators.
Three years ago, regulators rejected AT&T's $39 billion bid for T-Mobile US, which resulted in AT&T paying Deutsche Telekom, T-Mobile's full owner, a reverse break-up fee of $6 billion in cash and U.S. mobile assets.
T-Mobile shares closed up 6.5 percent at $32.94 on the New York Stock Exchange. Sprint shares were down 5.3 percent at $7.35. (Reporting by Edwin Chan in San Francisco, Supantha Mukherjee in Bangalore, Marina Lopes, Diane Bartz and Alina Selyukh in Washington, Harro Ten Wolde and Peter Maushagen in Frankfurt; Writing by Frank McGurty; Editing by Bernadette Baum, Greg Mahlich, Tom Brown and Andre Grenon)