In April, United Group received approval from the European Commission to acquire BTC for an undisclosed price.
However, no changes to BTC's management board were made and Viva Telecom Bulgaria still remains the company's sole owner, data entered into the commercial register last week shows.
Viva Telecom Bulgaria is fully owned by Luxembourg-registered InterV Investment.
Bulgarian business news outlet Capital.bg reported on Friday that United Group's acquisition of BTC has been completed.
"United Group has acquired another market leader in the region and through out combined expertise, experience and scale will offer the newest generation of telecommunications services and the best local and international content..." Capital.bg quoted United Group CEO Viktoriya Boklag as saying in a statement.
In November last year, United Group said that it has agreed to acquire BTC, which operates under the Vivacom brand, but did not disclose the price of the transaction. Subsequently, the company announced plans to place some 1.795 billion euro ($2.1 billion) worth of bonds to finance the acquisition and refinance existing debt instruments.
BTC, which has over 1.8 million customers in Bulgaria, competes with A1 Bulgaria, a unit of A1 Telekom Austria, and Telenor Bulgaria, a unit of Czech-owned PPF Group.
($ = 0.8438 euro)