The convenor of the Joint Action Committee of BSNL Associations and Unions VAN Namboodiri told a news agency that the strike began at 6 am at all centres except Delhi, where it started at 10 am.
The strike could affect calls to and from BSNL networks, he informed.
10,000 senior staff, and senior ITS cadre employees, who are not part of the strike, are working to make sure that the telecom services are not hit by the employees' strike, BSNL chairman and managing director Kuldeep Goyal said.
The strike was called for by a joint forum of unions that has both executives and non-executives.
The employees are unhappy with recommendations of the Pitroda committee suggesting voluntary retirement (VRS) to one lakh staff besides the 30 per cent disinvestment to improve financial health of the PSU.
Telecom Minister A Raja held talks with the union to avert the strike but failed in his attempt to persuade them to call off the strike.