Armstrong came to Verizon in 2015 following its takeover of AOL and headed Oath, which was created last year after the No.1 US telecom company acquired the core business of Yahoo and merged it with AOL.

Oath owns more than 50 brands, including HuffPost, TechCrunch and Tumblr, but has failed to make an impact in the space occupied by Facebook Inc and Alphabet Inc's Google.

At Verizon, Armstrong had been working on building AOL's expertise in placing text and video ads on mobile phones.

"Tim is as good as it gets in digital media, but the issue is less one of leadership than it is of the fit between digital advertising platform and a phone company," MoffettNathanson analyst Craig Moffett said.

"Phone companies are subject to stricter rules around data privacy - some legal, some self-imposed - than are companies like Google and Facebook. Unfortunately, that makes it harder for them to compete."

Armstrong headed AOL from 2009 to 2015, and was instrumental in the company reporting its first quarterly revenue growth in eight years in 2013.

"It's just a mismatch between a large, bureaucratic corporate culture and an entrepreneurial, decentralized, flat culture DNA of Tim Armstrong," said Raj Das at executive search firm Buffkin/Baker.

Last year, Verizon had shown interest in acquiring a significant part of Rupert Murdoch's Twenty-First Century Fox Inc's assets, which was finally sold to Walt Disney Co .

Verizon declined to comment, while Armstrong did not immediately respond to a request for comment.

Shares of Verizon were marginally down at $54.13 in afternoon trading.