The notice sent to Bharti Airtel, Idea Cellular and Vodafone asks them to explain why penal action should not be taken against them.
"We have been given 60 days to respond to the notice," a senior executive of one of the firms told IANS.
The operators had entered into roaming agreements to offer 3G services such as video calling, mobile TV and multi-media gaming in areas where they did not have 3G spectrum.
The DoT had last December asked the operators to terminate their 3G roaming agreements calling it illegal. The department had also said the government was losing revenue because of these agreements.
The operators had then moved the tribunal challenging the DoT order, saying it would harm customers and investment in the sector. In July, the tribunal delivered a split verdict.
The two members of the bench, comprising chairman Justice (retd) S.B. Sinha and member P.K. Rastogi, had differing views on the order. One favouring the DoT claimed the roaming agreements were illegal, while the other supported the operators who had challenged it.
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