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Commercial Information

 Commercial policy of COSMOS (Centrex Facility)                          
The numbering scheme for Centrex facility ‘city wide’

  •    In this scheme only 8 different-blocks are possible per Centrex Group and to match these with block numbers, we can have only 8 distinct levels for Areas.  Accordingly, 8 levels (2-9) have been allotted to Areas.  For Areas like LNG and SHD, SKN & RHN and JKP & KB common levels have been kept.  For rest of the areas, separate levels have been defined.

  •  The total no. of Centrex Group possible will be 1023.  Initially a block of 1000 numbers have been allotted for each Area Groups for allotment of new indicators.  Four digit number will be used for Internal dialing for Centrex Group. ‘O’ is used for PSTN access.

  •   Existing customer can opt for this facility in a group with minimum number of 5 members.  The upper limit of members of a Centrex Group is 8000 at present.  If a Group falls below 5,  it will  automatically be closed. However, they may continue as PSTN subscriber with change of numbers.

  •  The subscribers/customers can apply for this facility to CO(COSMOS) under DGM(Comml) HQ in the prescribed form as enclosed as Annexure II.  They will submit the copy of last paid bill alongwith the application in original.  There will be no signature verification at this stage.  However, in case of a company/Firm the authorized signatory will apply to CO(COSMOS) for Centrex facility.

  •  On receipt of application from a Group, the CO(COSMOS) will allot them a SOC name which can have 3 to 8 alpha numeric character and a SOC code of six digit.  The first two digits will indicate Area code and last 4 digit will indicate the Group code  For example, if Punjab National Bank applies for Centrex Facility, the CO(COSMOS) after verification of the identity of the authorized signatory will allot a Group name say ‘PNB’ to the Bank and if their main branch falls in Central area then he will have a SOC code like ‘04 0100’ (‘04’ for Area and ‘0100’ for  Centrex Group).  SOC codes 0000 to 0099 will be reserved for Areas for Centrex facility within Exchange Area / Wire Centre Area.

  •  Once a group is formed, any existing/new subscribers can become the member of the Centrex Group, later by applying to CO(COSMOS) along with their desired SOC code and CA No.

  •   A new customer who wants to have a new connection and also needs Centrex facility will apply for both NTC and Centrex Facility to CO(COSMOS).

  •   On Application of a Group, the CO(COSMOS) HQ will issue a single OB both for change of number and  provision of Centrex facility.

  •  Preferably every Group will be represented by a Group leader called Group Co-ordinator.  However, this will not be essential in every case.  Where a Group is formed with a Group-coordinator as their representative, then the co-ordinator can correspond with MTNL on behalf of the Group for getting the facility extended to a new subscriber.  However, there will be no interference of group co-ordinator in inclusion/exclusion of any Centrex Group subscribers.  Subscriber is free to enter/exit in Centrex Group.

  •    A uniform tariff of Rs.125/- per DEL per month will be charged for the COSMOS facility.  This would be in addition to the monthly rental chargeable from the subscriber as per his application tariff plan.  All intra COSMOS calls shall be free.

  • Telephone bills of the centrex customers will be issued by the concerned area.

  •  GM(Corporate Sales) will be in-charge of all Centrex customers. He will take care of the requirements of Centrex customers treating them as CIPs.

  •       Detailed procedure/Flow of OB for NTC, shift, change of  Nos. billing.  FRS, etc. will be issued separately by GM(IT).

CITYWIDE CENTREX OF MTNL DELHI(Technology and Numbering Scheme) 


  • OCB-283 New technology Switch is taken as source switch to provide the service. It is at JKPD4 with R-25 version.
  • OCB-283 has major deployment in the network. It forms 41% of basic services equipment with 27 main exchanges out of 65 and 80 RSUs out of total 144.
  • It has two stage remote switching capability called CSND and CNE. This provides scope to cover wire-centers widely .
  • It is easy, among all technology, to redeploys line units (CSND) from one place to another in existing network mainly due to simplicity and expertise available with project and maintaince unit as well.


  • There can be any number of Centrex subscribers within the limit of exchange traffic and connection capacity of the exchange.
  • Number of Centrex group can be 1023.
  • There are eight different blocks possible per Centrex group. Block number is designated by digit 2 to 9 (1 and 0 are not used. They are used for emergency/special service and PSTN access respectively)
  • Number of Subscriber possible per Centrex group is 8000.
  • Internal numbering or short code dialing ranges from two to four digits.
  • Block digit also forms starting digit of internal dialing.
  • Internal dialing digits, other than block number, are essentially last digits of PSTN number.
  • Example:  PSTN number is 28823546; Four-digit internal number for Centrex group is 7546.  

546 – three digits matching with PSTN number (000~999)  
7 – block number (2~9)  
28830000 – Head number (ND) for the block 7.

 Numbering Scheme

  • Scheme should be in such a way that four digits internal dialing code tallies with last four digits of PSTN number. Therefore fifth digit of number is to match with block number or prefix digit.
  • CSMS and FRS computer system at present can analyse up to fist four digits only, therefore fourth digit is chosen to  differentiate area. Care is taken to allot digit that tally with existing PSTN starting digit. Since due to limitations of ‘head number’ there can be eight series only, few geographically co-located area will share same exchange level though different range of last three digits.  
  • With above assumptions following  numbering scheme adopted 

288 Z Y XXX  
         |    |  |     |
         |    |  |     |
         |    |  |     |____
Centrex member number ( 000 to 999)
         |    |  |_________   
Fifth digit to match with prefix/block.
(2 ~ 9).
       |    |                    This again matches with ‘Z’
         |    |  
         |    |___________
Fourth digit to differentiate GM area(2~9)

2 – LNG(000~499)  & SHD(500~999)
|                   3 – East
|                   4 – Central
|                   5 – JKP(000~499) & KB(500~999)
|                   6 – BCP
|                   7 – SNG(000~499) & RHN(500~999)
|                   8 – RG
|                   9 –NP
|____________Centrex level for MTNL Delhi. Level 289  
reserved for expansion.

Based on above scheme, following are numbering chart for areas Central             -  28844000    to   28844999
East                 -  28833000    to   28833999
BCP                  -   28866000    to  28866999
NP                   -  28899000    to   28899999
LNG                 -  28822000     to  28822499
SHD                 -   28822500    to  28822999
SNG                 -   28877000    to  28877499
RHN                 -   28877500    to   28877999
JKP                  -   28855000    to   28855499
RG                   -   28888000    to   28888999

KB                  -   28855499    to   28855599

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