ShoreTel on Wednesday afternoon turned down Mitel’s latest unsolicited buyout proposal – ramped up to $8.50 per share on Monday from the original $8.10 per share.

ShoreTel called the offer “highly inadequate,” saying the bid undervalues its worth amid strong prospects for continued growth.

“After careful deliberation, ShoreTel’s board of directors has determined that the incremental value represented by Mitel’s revised, unsolicited proposal is highly inadequate, and that this revised proposal is again an opportunistic attempt to acquire ShoreTel,” said Chuck Kissner, chairman of ShoreTel’s board. “We continue to believe that Mitel’s highly inadequate proposal does not reflect the value inherent in ShoreTel’s business, nor does it reflect ShoreTel’s compelling prospects for long-term growth and value creation.”

Don Joos, president and CEO of ShoreTel, agreed.

“We are confident that executing our strategic plan is the best path forward and will deliver substantially more value to ShoreTel stockholders than Mitel’s significantly inadequate proposal.”

Blackstone Advisory Partners L.P. is serving as financial advisor, and Fenwick & West LLP is serving as legal counsel, to ShoreTel.

Mitel is keen to consolidate with ShoreTel to create one of the largest providers of IP business phone systems, cloud- and premise-based, although with emphasis on the services aspect that generates recurring revenue. There’s no word on whether Mitel will make a third attempt to purchase ShoreTel.