A Trai official told PTI that drive tests in seven cities across the country started on December 21 and are likely to end by January 8.
The cities are - Delhi, Mumbai, Surat, Kolkata, Pune, Bhubaneswar and Indore. Drive tests are conducted to assess coverage and quality of service of mobile networks.
Telecom Minister Ravi Shankar Prasad has said that call drop problem is improving and operators are putting up more tower sites.
The Minister last month said a total of 29,000 new telecom towers were installed by private telecom operators across the country after government took strong exception to the problem of call drops.
Operators have also said that they have taken steps to rein in call drops.
Meanwhile, Trai today told the Delhi High Court that "lack of investment" by telecom operators in network infrastructure, like mobile towers, appears to be the main reason behind the "pervasive problem" of call drops across the country.
In an affidavit placed before the court, the regulator said telecom majors, including Vodafone, Bharti Airtel and Reliance, "have failed to keep the investments commensurate with the pace of increase in usage and the growth in number of subscribers being added by them".
Trai's response came on the pleas filed by telcos challenging the regulator's order mandating them to pay consumers Re. 1 per call drop experienced on their network, subject to a cap of three a day.
Trai had carried out special independent drive tests in Mumbai and Delhi in June and July and found the network quality below par.
It followed up with more such tests in September, which showed there was not much headway.
Other than the metros, the tests had been conducted in Surat, Kolkata, Bhubaneswar and Ahmedabad. Trai Chairman R S Sharma had held a meeting with top bosses of telecom companies on October 29 and asked them to improve the quality of service.