According to an official, the last date to apply is November 24 and the selection of the firm is likely to be completed in two months.
"It will take two months to select the firm and the audit process may take about six months," the official added.
DoT had earlier conducted the audit for assessment years 2006 to 2008.
During that time, it had received complaints regarding some telecom companies under-reporting their revenues.
This time, DoT has not received any complaint regarding any company. However, exact reason behind the move could not be ascertained.
The operators challenged it in court though. The matter is sub judice.