Vivacom's main shareholder, Bulgarian businessman Spas Roussev, confirmed that a sale process was underway and that Lazard had been hired to handle it.
A minority shareholder, Russia's VTB Bank, said in a separate statement that bids have been sent out to potential buyers and feedback was expected by the end of the summer.
A consortium led by Roussev took control of Vivacom from Russia's VTB Bank after bidding 330 million euros ($371 million) for it in an auction in 2016.
The consortium also took on 450 million euros of Vivacom's senior debt at the time.
"Non-binding offers are expected by the end of the month," one source, who wished not to be named, said.
Another source familiar with the process confirmed the timeline and added that over a dozen interested parties have signed non-disclosure agreements and received a detailed investment memorandum.
According to market sources, Vivacom's enterprise value could exceed 1.2 billion euros ($1.35 billion), including net debt of about 600 million euros.
At present, Roussev has a 46 percent stake in Vivacom, VTB holds 20 percent minus one share, while a further 19 percent is held by Delta Capital, controlled by the managers of the Bulgarian office of VTB Capital. The rest is held by former creditors of Vivacom.
The auction sale in 2016, has been contested at British court by Empreno Ventures, controlled by Russian businessman Dmitry Kosarev, who bought a stake in a holding company that owned Vivacom prior to the auction. Kosarev claimed the auction deprived him of his investment.
Vivacom said the commercial court had dismissed Empreno's claims on technical grounds, and that the auction was legitimate and fair.
But in a letter sent to Lazard and seen by Reuters, Empreno said it was continuing to assert its rights over Vivacom's shares and was taking steps to launch new legal actions to reverse the auction, urging Lazard to inform potential investors.
Vivacom, with total assets of 1.7 billion levs ($978 million), ended 2018 with a net profit of 128 million levs, up almost 80pc from a year ago, its consolidated report showed.
Its adjusted earnings before interest, tax, depreciation and amortisation rose 12.8pc year-on-year to 360 million levs in 2018.