According to sources in the Central Bureau of Investigation (CBI), the matter came to light after information was received that calls were being made from the Mahanagar Telephone Nigam Exchange (MTNL) Prabadevi exchange to members of the underworld in Dubai and Karachi.

Investigations revealed that a telephone linesman had bypassed the exchange point, the pulse code modulator by cutting the main line and connecting 10 telephone lines to the main Trunk Automated Exchange (TAE) junction. The ingenius method not only concealed the identity of the receiving party but also bypassed the entire system of metering and monitoring telephone calls.

The linesman had actually connected the subscriber's line directly to the TAE, which was accessable to STD and ISD calls. Moreover, since thecalls were not metered, the caller could speak for an extended duration.

The ground and eighth floor of the Prabhadevi exchange were sealed this afternoon after a CBI officer found the trunk exchange tampered with. Two premises at Dadar were also raided from where the calls were suspected to be routed through the exchange.

Preliminary investigations reveal that the tampering of lines was being done since the last two months. Strangely, despite the exchange being fully automatic, officials had failed to detect the fault.

The lines were cut and wrapped in tape under the supervision of senior MTNL officials. Incidentally, the Prabhadevi exchange is the MTNL's headquarters in the city.

CBI sources said though it is premature to say how many calls were made and where, it is suspected that the caller and his associates were linked to the Dawood Ibrahim gang. The exact amount of loss to MTNL is yet to be ascertained, sources added.

The MTNL linesman has been identified as S C Shekar. CBI officials haveseized Rs 24 lakh from his residence, which according to them is disproportionate to his known sources of income.

The ingenius idea assumes significance in the light of the fact the police department has managed to record long conversations between a Sessions Court judge and underworld don Chhota Shakeel, based at Dubai. Under the system such calls could never have been detected, MTNL sources revealed.