BUZZ-ShoreTel Inc: Mitel Networks offers to buy co for $540 mln
** Communication equipment maker's shares up about 18 pct at $7.65 premarket
** Larger peer Mitel Networks Corp offers to buy ShoreTel for $540 million, or $8.10 per share
** Offer represents 24 percent premium to ShoreTel's closing price on Friday
** Mitel says ShoreTel had rejected an earlier proposal and refused to engage with Mitel
** Up to Friday's close, ShoreTel stock had fallen nearly 30 pct this year
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