Activist investor Elliott Management Corp. on Monday questioned AT&T Inc.'s $85 billion takeover of Time Warner and called for the wireless carrier to sell noncore businesses to boost its stock price, driving its shares their highest in a year.

Billionaire Paul Singer's hedge fund, which disclosed a $3.2 billion stake in AT&T, called the company "deeply undervalued" and hinted it would press for board seats, while proposing changes that could double the company's current share price.

"Despite nearly 600 days passing between signing and closing (and more than a year passing since), AT&T has yet to articulate a clear strategic rationale for why AT&T needs to own Time Warner," Elliott said in a letter to the company's board of directors.