These calls were handled by tandem exchange " Toll B ", from which the second dialling tone originated.
2.
But instead of calls being overflowed to main or tandem routes shared with other calls, the unsuccessful callers receive a reorder tone from their local or tandem exchange.
3.
Holborn was also a tandem exchange for calls to and from local manual exchanges, so that the tandem traffic from manual exchanges was combined with traffic from automatic exchanges.
4.
Handling local calls between manual exchanges via a third operator at a tandem exchange is uneconomic, but without tandem switching each manual exchange would require direct junctions to every other manual exchange.
5.
Direct or tandem junction routes to other exchanges could be allocated as required, with routing independent of the telephone number and able to be altered at any time to cater for traffic growth or the introduction of new local or tandem exchanges.
6.
It has got technology labs for broadband ( ADSL ), SDH, OFC, MPLS, ISDN, A / V, OCB 283, Digital UHF / MW, GSM, DWDM . The Centre has functional GSM / BTS & PSTN exchange model, and also organizes study tours to MTNL's TAX / TANDEM exchanges.
7.
In the London telephone area, calls to subscribers who were not serviced by Director telephone systems within local call-charging range of the London Director exchange area were carried by tandem exchange " Toll A " : a subscriber would dial a prefix code, usually three letters, followed by the number of the other subscriber on the fringe non-director exchange.
8.
Distant exchanges which did not justify direct junction routes could be called via one or more tandem exchanges; being routed via one, two or three local code selectors in the originating exchange, one or more selectors in the tandem exchange ( s ), and finally the numeric selection stages in the terminating exchange for the last four digits, which were stored and forwarded without translation.
9.
Distant exchanges which did not justify direct junction routes could be called via one or more tandem exchanges; being routed via one, two or three local code selectors in the originating exchange, one or more selectors in the tandem exchange ( s ), and finally the numeric selection stages in the terminating exchange for the last four digits, which were stored and forwarded without translation.
मोबाइल