Pesky calls from telemarketers may continue to nag customers for some more time as the Telecom Regulatory Authority of India is likely to extend its March 21 deadline again.

"We have not received the landline number series yet from the department of telecommunication.

"Unless that is given, it will be difficult for telemarketers to operate from mobile network. We have not decided the next date but actual date will be soon," Trai chairman J S Sarma told reporters in New Delhi.

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The deadline has already been extended three times.

The regulation was to be initially implemented on January 1, 2011.

It was later extended to February 1 and then a second extension to March 1 was made for implementing the regulation.

DoT has issued mobile numbers starting with '140' to help subscribers identify telemarketing calls.

Similarly, a series is to be allocated for landline numbers.

DoT sources say the delay in issuing the landline series is because there could be a security breach at telecom exchange level.

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"Most of the work in case of landlines is done manually.

Fixed line network is not automatic as mobile network is.

Identification in case of fixed line is done through manual process, a DoT official said.

At present, if any call from landline comes, then it can be identified based on STD code -- the initial digits -- and then levels (digits following STD codes) which differ from exchange to exchange within a city.

In case identifiable series of '140' is used, it will impact the numbering system being used at present for allocating new connections.

Also, it will be difficult for security agencies to track calls as it will display uniform code instead of STD code for connections across the country, the official said.