KKR & Co and Global Infrastructure Partners (GIP) are among the private equity firms vying for a stake in Vodafone‘s wireless tower unit, the people said.

Swedish investment firm EQT AB has also been exploring a potential investment in Frankfurt-listed Vantage Towers AG, the people said, requesting anonymity to discuss confidential information. The people said that Vodafone had invited suitors to the auction process.

Shares of Vantage rose 4.2% on Thursday, giving the company a market value of €13.4 billion ($13.4 billion).

Vodafone has yet to decide on the size of the stake it wants to sell, although some bidders are eager to secure a majority stake, the people said. According to data compiled by Bloomberg, the British airline owns about 82% of Vantage.

The people said that it is uncertain whether the private equity firm will make a formal offer, and other bidders could still emerge.

Phone operators in Europe have started selling infrastructure assets to raise money to invest in expensive fibre deployments and wireless network upgrades and to pare down their massive debt. In July, Deutsche Telekom agreed to sell a majority stake in its towers unit to Brookfield Asset Management Inc and DigitalBridge Group Inc in a deal that valued the business at €17.5 billion.

Operators once viewed these assets as critical to their business models and attractive to investing companies due to their stable, predictable returns. KKR raised $17 billion for its latest global infrastructure fund earlier this year. In February, Bloomberg News reported that GIP aimed to raise $25 billion for the world’s largest infrastructure investment fund.