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.
Operators have also said that they have taken steps to rein in call drops.
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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 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.