Calcutta Telephones has decided to upgrade its technology and change all existing seven-digit numbers to eight, starting December 1.

“The changed numbering system will be introduced in all city telephone exchanges. The first digit will be 2, followed by the existing seven digits,’’ chief general manager S.P. Chakraborty announced on Tuesday. “Once regional telecom heads adopt the technology, all BSNL numbers will start with 2,’’ Chakraborty added.

Telephone officials explained that both BSNL and MTNL have adopted 2 as their ‘base number’ for the country. Private telecom operators have a different base number, they clarified.

Calcutta Telephones had switched from six-digit to seven-digit numbers in 1997.

Officials said the telecom authorities in London have introduced the eight-digit number for technical reasons. “The response from subscribers has been extremely positive,’’ a senior telecom official here said.

Ultimately, added the official, a uniform, countrywide, 10-digit number system will be introduced by both BSNL and MTNL.

Elaborating on this, Chakraborty said the area code would then be added to the local number. For example, 33 is the area code for Calcutta, which would be added to an eight-digit number. Haldia’s area code is 3224 and so, once the West Bengal Telecom circle adopts the new technology, the local Haldia number will be made six digits to get the arithmetic right.

Chakraborty said it was necessary to upgrade technology to cater to a growing subscriber base. “The number of digits has to be increased to keep pace with the technology,’’ he said, pointing out the following advantages of the new system:

lClarity in voice transmission

lImproved connectivity for STD and ISD callers (elimination of the prolonged beep or pause that often greets callers). The speed of connectivity and quality would both improve

lReduction in the level of switching faults in the internal circuit

Chakraborty clarified that as “levels 0 for STD and 00 for ISD are access codes, they will have nothing to do with the numbering system’’.

In case of the wireless on local loop, WLL mobile, the first digit of the existing telephone number, that is 7, will be replaced by 21. For example, 7009222 will become 21009222.

Telecom officials said the Central communication ministry has set a target of connecting a crore homes by the end of this decade. Union minister Pramod Mahajan held a series of meetings on this with the MTNL and BSNL chiefs and regional heads last month. Subsequently, the BSNL and MTNL authorities directed their regional heads to take immediate steps to upgrade their switches.

Admitting that the December switch would cause some confusion, Chakraborty said a software was being installed to inform callers about the changed number.