1. A telecommunications system comprising a plurality of local exchanges in which signalling and traffic data are routed between remote exchanges via a network service having routing code recalculation, which network service includes a plurality of transit nodes, said telecommunications system being adapted to provide a supplementary subscriber service involving an interaction between two subscribers in which:', '- knowledge of a subscriber access code, used in a called subscriber local exchange, is required in a calling subscriber local exchange;', 'said subscriber access code is generated by modification of a called subscriber routing code, used by said called subscriber local exchange; and', 'said modification occurs in a transit node of said network service,', 'each local exchange having an applications service element for providing said supplementary service, said telecommunications system being characterised in that at least some transit nodes include a relay applications service element which is adapted to relay signalling data between local exchanges and which includes translation means for converting a subscriber access code used at a called subscriber local exchange into a called subscriber routing code used by a calling subscriber exchange, and in that signalling data is transmitted between said calling and said called local exchanges via said relay applications element in said', 'SUBSTITUTESHEET transit node.

2. A telecommunications system as claimed in claim 1 characterised in that one, and only one, transit node through which signalling is transmitted between a called', "5 party's local exchange and a calling parties local exchange includes said relay applications service element, so that, in use, a two stage TC chained signalling dialogue is established between said called party's local exchange and said calling party's local", '«. exchange.', "3. A telecommunications system as claimed in claim 1, characterised in that two transit nodes through which signalling is transmitted between a called party's local exchange and a calling parties local exchange include", "15 said relay applications service elements, so that, in use, a three stage chained TC signalling dialogue is established between said called party's local exchange and said calling party's local exchange.", '4. A telecommunications system as claimed in any of 20 claims 1 to 3, characterised in that said supplementary subscriber service is a CCBS service.

5. A telecommunications system as claimed in any of claims 1 to , characterised in that said network service having routing code recalculation is a GVNS zb service.

6. A telecommunications system as claimed in any of claims 1 to 5, characterised in that said telecommunications system is a single unitary ISDN system.

8. A telecommunications system as claimed in any of claims 4 to 7, characterised in that each of said plurality of transit nodes which is adapted, in use, to receive signalling from a local exchange, providing CCBS to subscribers connected thereto, has a relay CCBS-ASE located therein.

9. A telecommunications system as claimed in claim 8, characterised in that said relay CCBS-ASE is adapted to change the called party code contained within a received TC signalling dialogue and to perform no other CCBS-ASE function

10. In a telecommunications system comprising a plurality of local exchanges in which signalling and traffic data are routed between remote exchanges via a network service having routing code recalculation, which network service includes a plurality of transit nodes, a method of providing a supplementary subscriber service involving an interaction between two subscribers in which:', 'knowledge of a subscriber access code, used in a called subscriber local remote exchange, is required in a calling subscriber local exchange;', 'said subscriber access code is generated by modification of a called subscriber routing code, used by said calling subscriber local exchange; and', 'said modification occurs in a transit node of', 'SUBSTITUTE SHEET said network service,', 'each local exchange having an applications service element for providing said supplementary service, said method being characterised by:', '- transmitting signalling data from said called local exchange to a relay applications element located in a transit node, said data including the subscriber access code used at said called subscriber local exchange;', '- converting said subscriber access code to said subscriber routing code, in said transit node;', 'modifying said signalling data received from said called subscriber local exchange by substitution of said subscriber routing code for said subscriber access code; and', 'transmitting said modified signalling data to said calling subscriber local exchange.', "11. A method as claimed in claim 10 characterised by transmitting signalling data between said called party's local exchange and said calling party's local exchange in a two stage TC chained signalling dialogue, the first stage of which is between a transit node and said called party's local exchange, and the second stage of which is between said calling party's local exchange and the transit node.", "12. A method as claimed in claim 10, characterised by transmitting signalling data between said called party's local exchange and said calling party's local exchange in a three stage TC chained signalling dialogue, the first stage of which is between a first transit node and", "SUBSTITUTESHEET said called party's local exchange, the second stage of which is between first and second transit nodes, and the third stage of which is between said calling party's local exchange and said second transit node."13. A method as claimed in any of claims 10 to 12, characterised in that said supplementary subscriber service is a CCBS service

14. A method as claimed in any of claims 10 to 13, characterised in that said network service having routing code recalculation is a GVNS service

15. A method as claimed in any of claims 10 to 14, characterised in that said telecommunications system is a single unitary ISDN system

16. A method as claimed in any of claims 13 to 15, characterised by transmitting said signalling data between CCBS-ASEs located in each local exchange, within said telecommunications system, providing CCBS to subscribers connected thereto

17. A method as claimed in any of claims 13 to 16, characterised by processing said signalling data received at each of said plurality of transit nodes from a local exchange, in a relay CCBS-ASE located in each of said transit nodes

18. A method as claimed in claim 17, characterised by, in said relay CCBS-ASE, changing the called party code contained within a received TC signalling dialogue and performing no other CCBS-ASE function on said signalling data

19. A network service transit node for use with the method as claimed in any of claims 10 to 18,', 'SUBSTITUTE SHEET characterised in that said transit node includes receiving means for receiving signalling data transmitted from a called local exchange, said data including the subscriber access code used at said remote local exchange, converting means for converting said subscriber access code to a subscriber routing code; signal generating means for generating modified signalling data, from signalling data received by said receiving means, by substitution of said subscriber routing code for said subscriber access code; and transmitting means for transmitting said modified signalling data to said calling subscriber local exchange.

20. A network service transit node as claimed in claim 19, characterised in that said converting means is a relay CCBS-ASE.

21. A telecommunications system as claimed in claim 20, characterised in that said relay CCBS-ASE is adapted to change the called party code contained within a received TC signalling dialogue and to perform no other CCBS-ASE function.', 'SUBSTITUTESHEET']

30 7. A telecommunications system as claimed in any of claims 4 to 6, characterised in that a CCBS-ASE is', 'SUBSTITUTE SHEET located in each local exchange within said telecommunications system providing CCBS to subscribers connected thereto.