Bill Ackman's Pershing Square has been building a stake in United Technologies, the investor told CNBC.

Ackman said he thought it was a "great" company in a phone conversation with CNBC's Scott Wapner.

The company's shares closed up 0.6 percent Wednesday.

United Technologies' chief executive said the defense contractor is thinking of splitting up key parts of its business at a conference last week. The comments spurred a rally in the company's shares the following day.

"Is [United Technologies] a more valuable property together or is UTC better off in three separate businesses?" CEO Greg Hayes said at the Barclays Industrial Select Conference in Miami on Feb. 21. "That's the question for the board. That's the question we continue to study."

"There are, as you can imagine, significant dis-synergies with splitting up the portfolio, as well as one-time costs," Hayes added.

In January, Reuters reported Bill Ackman is planning to lower his public profile after several years of disappointing returns. The hedge fund's returns fell 4 percent in 2017, dropped 13.5 percent in 2016 and declined 20.5 percent in 2015, according to its website. The fund is also down 5.6 percent year to date as of Feb. 20 this year.

United Technologies declined to comment.

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