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America Movil (AMX) said on Friday it wanted to buy the 70 percent of KPN that it does not already own, in a challenge to its Spanish arch-rival Telefonica which last month offered to buy KPN's German business E-Plus - its crown jewel - for $11 billion (7 billion pounds).
The statement from the foundation, which represents the interests of KPN owners, employees and customers, could be the first sign of opposition to Slim's bid.
KPN's board, which has shown little enthusiasm for the billionaire's involvement in the company since he built up an almost 30 percent stake last year, has yet to say whether it supports America Movil's offer.
Foundations, a common and independent element in many Dutch companies, can block hostile takeovers in the Netherlands.
The foundation would typically exercise a call option, triggering a huge issue of preference shares from the target company and diluting the holding of an unwelcome or hostile bidder.
"Does this mean we will exercise our call option? No, that's not being considered now. But it's important to note that we have that right," a spokesman for KPN's foundation said on Wednesday.
KPN's shares fell 3.4 percent to 2.26 euros on Wednesday, well below the 2.40 euros that America Movil is offering - an indication investors think the offer may not succeed.
A move by the foundation to block America Movil's bid would likely "at least delay the company gaining full control of KPN," ratings agency Fitch said on Friday.
However, such blocking tactics can be challenged.
"If KPN asks the foundation for help, there's a big chance that this is going to the courts and then a judge could invalidate such a move," analyst Jos Versteeg at private bank Theodoor Gilissen said on Wednesday.
"There is considerable uncertainty about América Móvil's intentions," the foundation said late on Tuesday, citing AMX's brief announcement that it intends to make an offer and the lack of information about its position on KPN, "including KPN's intention to sell its German subsidiary E-Plus."
Carlos García Moreno, America Movil's chief financial officer, this week told Dutch media that the Mexican group wants to "unlock value" at KPN and is not just interested in E-Plus.
KPN has been struggling to reverse a decline in revenues and profit in the face of stiff competition at home, where it still has a market share of about 45 percent in fixed-line and mobile telephony.
More competition is on the horizon in the Netherlands, as Nordic telecoms group Tele2 said it would offer low prices for mobile internet to grab a significant part of the Dutch mobile market.
(Reporting by Sara Webb; Editing by Erica Billingham)
By Sara Webb and Robert-Jan Bartunek
