NEW DELHI: Telecom operator Tata Teleservices is believed to be in talks to sell its entire stake in mobile tower company Viom Networks to American Tower Corporation for about Rs 4,000 crore."Tata Teleservices is in advance stages of finalising deal with American Tower Corporation (ATC) to exit Viom Networks. The deal is pegged at around Rs 4,000 crore," an industry source said.Mails sent to Tata Teleservices and ATC did not elicit any response.Tata Teleservices' stake in Viom Networks came down to 26 per cent after ATC acquired majority stake in the company and merged its existing Indian business.ATC bought 51 per cent stake in Viom, which owns and operates over 42,000 towers, from Tata Teleservices Ltd and SREI Infrastructure Finance for Rs 7,635 crore.Tata Teleservices at that time owned 54 per cent of Viom, with Kolkata-based SREI Group of Kanoria family holding about 18 per cent with management control.Singapore state investor GIC, Macquarie SBI Infrastructure Fund, Oman Investment Fund and India's IDFC Private Equity are other investors in the company.Viom was formed in 2009 as a result of merger of mobile towers of Tata Teleservices and Quippo Telecom -- a unit under SREI Group.