Union sources said they were informed by the company, taken over by Patrick Drahi in 2014, that a third of its workforce in France will be cut by 2019.
Move by Patrick Drahi, owner of Numericable-SFR, to buy rival Bouygues Telecom for 10 billion euros is set to provoke controversy.
About four million SFR clients lost their connections over several hours on Thursday, caused by the malfunction of a 'home location register'.
Decision casts doubts over immediate prospect of reducing number of competitors in the French market in bid to end a bitter two-year price war.
The former France Telecom's move comes as French ministers encourage 'consolidation' in telecom industry from four to three major players.
After a month-long fierce bidding war against Bouygues, Numericable buys Vivendi's telecoms subsidiary SFR for 13.5 billion euros.
Head of French public-sector investment body says he would be prepared to back Bouygues in tussle for control of mobile phone firm SFR.
Move is blow to telecoms firm Bouygues who had also bid for its mobile phone rival, an option also backed by industry minister Arnaud Montebourg.
Some observers suggest that the return to just three main mobile phone operators will reduce competition and be bad news for French consumers.
Bouygues Telecom's offer to sell its mobile network to Iliad, owner of Free, is dependent upon regulators approving its bid for Vivendi's SFR.
If accepted, Bouygues' offer for SFR would reduce the number of players in France’s ferociously competitive mobile market from four to three.