Members of the Communication Workers Union in Bury, Glasgow, Leeds and Preston Brook, near Warrington, will vote over the next few weeks on whether to launch a campaign of industrial action.
The union said there were rumours of site closures and off-shoring work to South Africa and India.
Andy Kerr, CWU deputy general secretary, said: "This ballot is about protecting decent UK jobs. We've been forced to hold a ballot because of the unusually short consultation period and lack of progress on assurances over job security, terms and conditions, and the future of sites.
"We wanted to have a negotiated way forward but our hands have been tied. We're already two weeks into the 41 day consultation period and we've made no progress at all on key issues.
"Trust is at rock-bottom, made worse by media speculation around site closures and offshoring work. Our members don't know who to trust and we feel like we've been lied to. We've been left no option by the company but to hold this ballot."
The ballot result is due on June 18.
An O2 spokesman said: "We are disappointed. We're committed to meeting the CWU as frequently as necessary to discuss their concerns with the aim of reaching agreement and avoiding the ballot and any subsequent industrial action."