Andy Kerr, CWU deputy general secretary, said: "We hope that any takeover of Telefonica UK will protect UK jobs and maintain service standards for customers. O2 has been through a great deal of operational change in the last two years with the biggest outsourcing deal in the UK when staff and services were outsourced to Capita.
"Staff working for both O2 and Capita will be concerned about what these latest ownership talks mean for their jobs and the future of the company.
"We will be meeting with the CEO where we will be seeking assurances over the long-term job security of our members in Telefonica UK. We will also be seeking a meeting with Capita to discuss job security for those workers who have recently been outsourced."
CWU has been assured by Telefonica UK that the union will be involved in talks and a meeting has already been scheduled for CWU officials to meet CEO Ronan Dunne on 2nd February.
CWU represents over 3,400 staff working in call centres, support and engineering roles across O2 and the associated Capita workforce.