"OTT players will have to roll out separate packages for VoIP (Voice Over Internet Protocol) services which shall eventually be charged to the subscribers," India Ratings and Research said in a note today.

"Players such as Whatsapp, WeChat, and Hike shall have to tweak their business models by differentiating between the messaging and VoIP product offerings," it added.

Last month, the official panel on net neutrality had proposed regulation of domestic calls on Internet-based apps like Skype, WhatsApp and Viber by putting them on par with services offered by telecom operators.

"The DoT committee's recommendation of regulating only the domestic calls of voice over internet protocol (VoIP) service providers is aimed at protecting voice revenues of telecom service providers (TSPs)," Ind-Ra said.

Conventional voice calls get a revenue of 25 paise per minute for the telcos as against the 6 paise per minute, it added, stressing that the messaging services offered by OTTs have already dented SMS revenues for the companies.

It said that voice services still command 50 percent of revenues for telcos, even though there has been a decline in average revenue per user on voice recently. The ARPU (Average Revenue Per User) for voice dipped to Rs 68.3 per month in December 2014 from the Rs 71.7 per month in July same year, the credit rating firm added.