Telegram's new free voice calls feature has been blocked in Iran, days after it was released in the country.

The encrypted app rolled out its feature to users globally, but the company says mobile operators and home internet providers have blocked the new feature in Iran. More than 20 million Iranians use Telegram, according to the country's Supreme Council of Cyberspace.

"We did not receive any communications regarding the reasons, so we have no idea what could stand behind the decision," Telegram said in the statement. "However, the fact that other voice over internet protocol (VoIP) providers in the country are still operating normally clearly suggests that this move is targeting Telegram specifically."

Iran's Minister of Information and Telecommunications, Mahmoud Vaezi says the block is a result of a judicial ruling issued to all phone operators. However, other VoIP providers like WhatsApp, iMessager, and Skype have voice call features enabled in the country.

Telegram claims users in Iran can still try to use the service, provided both ends use encrypted networks. Meanwhile, Twitter and Facebook continue to remain banned and can only be accessed by using a VPN.

Telegram's popularity is credited to its emphasis on security, and its ability to allow users to conduct "secret chats".

"Telegram is providing a service that cannot as directly be surveilled as normal telephone calls," Collin Anderson, an independent researcher focused on Iranian cyber policy told Bloomberg. "These restrictions are often economically motivated – the applications cut into the revenue generated by charging for text messages and airtime," he said.