One person has been arrested in Mumbai so far. The probe, sources said, is underway to ascertain the identity of other individuals involved in the network and locations of other similar exchanges. Sources said more arrests are expected in the next few days.
Official sources said that in a joint operation, the crime branch of the Mumbai Police and the military intelligence of the Indian Army unearthed three functional Chinese SIM boxes and one standby sim box along with 191 SIM cards, laptop modem; antennas; batteries and connectors used for an illegal VoIP exchange in Mumbai.
VoIP is the transmission of voice and multimedia content over Internet Protocol (IP) networks.
The lead for the spy network operating using a VoIP exchange came in May earlier, when defence persons received calls from suspicious numbers, seeking information related to Ladakh region and important defence installations.
Incidentally, for over a week, Indian and Chinese soldiers have been locked in a face-off along the Line of Actual Control (LAC) in Eastern Ladakh. The situation has been so tense that the US President Donald Trump offered to arbitrate between the two sides. Both the countries, have however, rejected his offer and are using established bilateral mechanisms to resolve the issue.
Since the callers were using fictitious identity, the investigating agencies suspected the role of the ISI, Pakistan's spy agency. Further investigation by the two agencies revealed that a few illegal VoIP exchanges at Mumbai, which routed calls incoming from Pakistan to local numbers, were used to extract information from defence persons.
"Based on this information, the crime branch and the military intelligence carried out a joint operation," sources said.
"The exchanges which converted international voice calls originating from Pakistan into local GSM calls using the Chinese SIM Boxes (boxes fitted with GSM SIM cards of local cellular service providers) were busted in a police raid on Thursday. These SIM boxes also used dynamic IMEI system, which made them difficult to be tracked," sources said.
This system is declared illegal by Telecom Regulatory Authority of India.
Official sources said the subsequent probe revealed that these illegal exchanges were being "used by hostile intelligence agencies to seek military information through calls," posing serious security threat to the country.
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