Cloud communications provider Mitel Networks (MITL) is increasing the pressure on fellow communications systems provider ShoreTel (SHOR) to accept its acquisition proposal by upping its bid to $8.50 per share, or about $574 million.

Mitel’s latest offering consists of a proposed $8.10 in cash per outstanding ShoreTel share and an additional 40 cents per share in Mitel common stock, which equates to 2.72 million shares. The bid also represents a 31 percent premium to ShoreTel’s unchanged stock price as of Oct.17 and a 38 percent premium to ShoreTel’s enterprise value, according to the company.

The latest offer comes just weeks after ShoreTel’s Board of Directors unanimously refused Mitel’s proposed buying price of $8.10 per share, or $540 million, claiming the offer undervalued the company’s prospects.

The ShoreTel board is advising its stockholders not to act on the new proposal until it has been reviewed thoroughly.

Mitel has steadily been increasing the pressure on ShoreTel’s Board of Directors since it first expressed interest in acquiring the company Oct. 2. This week’s proposal marks the second offer since the original price set forth in October, as well as Mitel CEO Richard McBee’s third letter to the ShoreTel board in less than six weeks.

“We continue to believe that the combination of our two companies offers a compelling opportunity to add sustained value to both organizations, and to solidify our combined leadership position in a highly competitive and rapidly consolidating market,” said McBee, in the letter to the ShoreTel Board of Directors. “In addition to the obvious benefit of an immediate and significant premium for ShoreTel shareholders, a combination of our two companies would create far superior value than could reasonably be obtained by ShoreTel as a standalone entity.”

ShoreTel’s Board of Directors has until Nov. 20 at 5 p.m. Eastern to respond to Mitel’s proposal.

Both companies have increased their revenues significantly over the past several months, with Mitel more than doubling its revenue in Q3 2014 over the previous period last year. ShoreTel, meanwhile, announced a 7 percent year-over-year increase for its Q1 2015 revenue.

Mitel recently rebranded its operations to focus more heavily on global expansion, with McBee expressing a keen interest in utilizing ShoreTel to push the company’s growth even further, according to our sister site Talkin’ Cloud.

With Mitel clearly eager to seal the deal with ShoreTel, it wouldn’t be much of a surprise if ShoreTel refuses this latest offer in the hopes of getting a better deal.