Idea Cellular has posted a loss of Rs 1107.
The two companies have also said they will look at selling their holdings in Indus, the country's biggest mobile masts operator with 123,000 towers, which is jointly owned with an affiliate of rival network operator Bharti Airtel.
Morgan Stanley was the financial adviser to Vodafone while DSP Merrill Lynch was the financial adviser to Idea Cellular.
ATC Telecom is a network of 20,000 towers across India.
Meanwhile, Idea reported a wider-than-expected second-quarter loss, citing the competition and the roll out of a new nation-wide Goods and Services Tax that pegged a higher rate on telecom services.
American Tower, which agreed to take control of Indian masts operator Viom in 2015 for 76 billion rupees, has been steadily expanding its presence in India - the world's second biggest wireless market with 1.2 billion mobile subscribers.
The deal is expected to be completed by the first half of 2018. Accordingly, private equity funds KKR and Canadian Pension Plan Investment Board (CPPIB) had emerged as the top contenders.
Shares in Idea fell 3.1 percent on Monday morning following the quarterly results.
The scrip ended at Rs 93.55, down 3.61 per cent on BSE.
"After Vodafone India and Idea have completed their merger, ~6,300 co-located tenancies of the two operators on the combined standalone tower businesses will collapse into single tenancies over a period of two years without the payment of exit penalties", according to an official statement read.