Under the so-called insourcing move, the staff currently working for Securitas at 68 of BT's main buildings across Britain will move to the company's facilities services arm, BTFS, when the current contract expires at the end of this month.
BT said the move will help deliver savings, which will be re-invested in facilities management services.
Neil Edmond, operations director for BTFS, said: "The front desk security and reception teams based at many of BT's offices have done a fantastic job, delivering an excellent service. We're delighted to be rewarding them with roles at BT.
"By bringing these employees into BTFS we're able to make savings which allows us to reinvest in the business."
Andy Kerr, Communication Workers Union deputy general secretary, welcomed the announcement, adding: "Ever since we became aware of the contract review we have lobbied for the insourcing option so this is a real success.
"The staff will now benefit from increased job security and possible career progression within BT Facilities Services."