Thirteen illegal telephone exchanges that were depriving the government of revenues of millions of rupees from international calls have been uncovered by telecom authorities.

The Vigilance and Telecom Monitoring Cell of the Department of Telecom and the Commissioner's Task Force of Andhra Pradesh Police busted the illegal telephone exchanges that had been operating in Hyderabad for the past year. Two people were arrested during the raids conducted Wednesday .

The racket was unearthed when authorities conducted raids in three zones of the city. The vigilance cell, while monitoring VSNL Internet ports, identified that a few subscribers were making heavy usage of Internet services according to a communications ministry press release Thursday.

Further investigations revealed that a set of subscribers was using the Internet to illegally push international calls into the local network.

These users ran set-ups that received international calls through the voice over Internet protocol (VoIP) route from the worldwide web and then distributed them through local numbers, preferably mobile phones.

"Due to this, international calls received in India registered as local calls," said a telecom official.

"These illegal set ups, besides causing huge revenue losses, were also posing a threat to national security as international calls handled by these set-ups could not be monitored," said the official.

The extent of the losses is still being calculated but is believed to run into millions of rupees, officials said.

These illegal networks consisted of sophisticated telecom equipments like VoIP gateway routers, multiplexes, modem and mobile phone equipment. Police have seized the equipment, including SIM cards.