The demand notices that have been sent out pertain to Delhi telecom circle, sources said, adding that this will be followed up by similar notices for other circles over the next fortnight.
The notices have given out for one circle for one year for each operator, sources added.
The six service providers who came under the spotlight following CAG report included Bharti Airtel, Vodafone, Aircel, Reliance Communications, Tata Tele and Idea.
Earlier this month, amid allegations and counter- allegations between BJP and Congress, DoT came out with a statement saying that it had received the key documents scrutinised by the CAG in mid-June 2016.
"These are being vigorously examined and the process of issue of demands for the four financial years for six operators in 22 licence service areas in consonance with licence agreement is currently ongoing," DoT had stated.