By David Gaffen (Reuters) - A flurry of bullish bets on Nokia just before markets closed on Friday are in line to pay off handsomely as a result of Microsoft's move to buy the Finnish company's handset business, and has once again raised questions about suspicious timing of such trades. Microsoft Corp said Monday that it would buy Nokia's handset business for $7.2 billion in a bid to win market share in the mobile business. The announcement sparked a massive rally in Nokia's U.S.-listed shares, which were up 30 percent on the day. But on Friday buying in call options, often used to express optimism about a stock in the near future, was far heavier than normal, prompting options strategists to note that an investor or investors benefited heavily from their timing. "On the surface it seems like one of those where someone probably knew something," said Ryan Detrick, chief technical strategist at Schaeffer's Investment Research in Cincinnati. "Regardless, there's a very significant winner in a couple of hours of market time there." About 18,700 September call option contracts traded on Friday, compared with just 1,143 put contracts, according to OCC, which clears all U.S. options. A call option gives the buyer the right to purchase a stock by a given date at a specific price. That lopsided ratio in favor of bullish bets compares with the rest of August, when on average about 3,700 call options and 1,800 put options traded on a daily basis, according to OCC. The leap in activity was all the more notable for its occurrence on the Friday before the U.S. Labor Day holiday weekend, a session that typically features some of the lowest trading volumes of the year. The most actively traded call options on Friday were in the September contract with a $4 strike price. About 11,000 in new contract positions were opened on Friday at an average cost of 14 cents, or about $154,000. Those calls now have a value of $1.18 million, according to Henry Schwartz, president of Trade Alert, who tracks unusual options activity. "This one stands out in terms of its size and the short-term nature of it," said Schwartz. Schwartz cited four separate late-day buys of 1,000 September contracts at a $4 strike price at about 14 cents per contract. The shares closed at $3.90 on Friday and were traded at $5.05 on Monday, a 30 percent gain. However, for the options, the gain was even greater: the $4 strike was traded at $1.07 per contract, a gain of about 760 percent, or about $372,000 on an initial cost of $56,000. The two companies have long-standing ties, however, and Detrick noted that rumors about the two have been bandied about in the past. In 2011, Nokia's Canadian head Stephen Elop, a former Microsoft executive, decided to use Microsoft's Windows Phone for its smartphone platform, rather than its own software or Google's Android operating system. The company had 40 percent of the handset market in 2007 and now just has 15 percent - and just three percent in smartphones. The stock has been in steady decline since its 2007 high of $41.10 as its market share dissipated. A spokeswoman for the Chicago Board Options Exchange, which coordinates efforts for the Options Regulatory Surveillance Authority, said: "CBOE takes its regulatory responsibility very seriously and does investigate unusual trading activity, however, we do not comment on individual investigations." A spokesman for the U.S. Securities and Exchange Commission, which also investigates unusual trading, declined comment. Representatives for Microsoft and Nokia were not immediately available for comment. Two studies done for Reuters found there were numerous examples of unusually heavy options trading prior to market-moving news like mergers or strategic acquisitions. (Reporting By David Gaffen)