Idea is much smaller than Bharti in revenue terms, but both mobile carriers operate on the GSM technology and have similar kinds of subscribers.
Kotak Institutional Equities said in a report that Idea's voice traffic declined 5.8 percent sequentially compared to its estimate of a 3.5 percent decline.
"Idea's voice traffic growth is disappointing. Part of this can be seasonality, but other underlying reasons are yet to be confirmed. And this is getting extrapolated to Bharti's Indian operations as well," said an analyst tracking the sector at a foreign brokerage house.
Bharti Airtel shares were trading at 344 rupees, down 1.5 percent while Idea shares fell 2.5 percent at 1051 am.