NEW YORK (Reuters) - Some of Frontier Communications' largest shareholders are concerned about its planned $9.6 billion takeover by Verizon Communications, with its second-largest investor planning to vote against the deal, three sources sources familiar with the firm's plans said.

Glendon Capital Management, which owns nearly 10% of Frontier, believes Verizon's $38.50 per share offer is too low, the people said. With acquired debt, the deal would be worth $20 billion.

The investor plans to vote against it when the deal comes up for shareholder vote on Nov. 13, the sources said. A majority of outstanding shares need to vote in favor of the deal to be approved.

Separately, Cerberus Capital Management, which owns 7.3% of Frontier, has privately expressed its view that the Verizon purchase price dramatically undervalues Frontier, people familiar with the investment firm's thinking said. It was not immediately clear how the investment firm would vote. A spokesman for Cerberus declined to comment.

Verizon and Frontier did not immediately respond to a request for comment. The sources requested anonymity to discuss internal deliberations.

When the deal was announced last month, it represented a 44% premium to Frontier's 90-day volume-weighted average share price. Verizon CEO Hans Vestberg called the acquisition "a strategic fit" that would allow the company to be more competitive in additional markets. Management has said the deal could take 18 months to close.

Frontier's stock closed at $35.25 on Monday, more than $3 below the proposed deal price.

Verizon announced the deal almost a year after activist investment firm Jana Partners said it had built a position in Frontier and was calling on the third-largest U.S. fiber broadband provider to sell itself.

For Verizon the acquisition would help it compete better against rivals AT&T and T-Mobile as they double down on unlimited plans and bundling options.

The investors' views come as some research analysts have also said Verizon's price is low and that investors should wait because Frontier's assets will become more valuable over time.

"We think investors should refuse to vote in favor of the deal unless they receive a higher price," New Street Research analyst Jonathan Chaplin wrote in a report last week. The report said Verizon could "comfortably pay at least $67 and still create value for its shareholders."

Ares Management, Frontier's biggest investor with a 15.6% stake, declined to comment on its views about the price or how it may cast its vote next month.