AN IMPLEMENTATION OF CALL DROP-BACK IN A TELECOMMUNICATION NETWORK

Abstract: The present invention proposes a method for performing a call drop-back in a telecommunication system, said telecommunication system comprising at least one exchange (MSC) for switching calls between at least two subscribers (A, C) or between one of said subscribers and a message server (VMS/SN), wherein said message server (VMS/SN) is adapted to maintain a record of a plurality of previous messages intended for a called subscriber (A) and which previous messages were recorded to the message server record from a respective calling subscriber (C), wherein said method comprises the steps of: invoking (S6) a callback call from said called subscriber (A) to said calling subscriber (C) based on the information recorded in said message server record via at least one of said at least one exchange (MSC), and upon invoking said callback call, setting the system into a state enabling said called subscriber (A) to be connected back to said message server (VMS/SN) after the termination of said invoked callback call. Also, a corresponding control device as well as a network element comprising the same are proposed.

Description: An imp] ementation of call drop-back in a telecommunication network FIELD OF THE INVENTION The present invention relates to a method for performing a call drop-back in a telecommunication system and to a corresponding control device as well as to accordingly adapted telecommunication network elements provided with the same. BACKGROUND OF THE INVENTION Recently, the features offered by telecommunication networks in connection with so-called value added services have remarkably increased in number. Among such features, there has been established a feature permitting a calling subscriber C, calling a called subscriber A (who, at the moment of being called, is not reachable for whatever reason) , to leave a message on a message server for the called subscriber A. A message thus left on a message server is also referred to as a message registered to said message server. The called subscriber A may listen to the message left for him (and/or thus registered to the message server) at an arbitrary point of time later. Thus, such a message server has a function of an answering machine provided at the telecommunication network side. For the purpose of the present invention, the following should be noted. A telecommunication system means any telecommunication in which at least one exchange for switching calls between at least two subscribers or between one of said subscribers and a message server is provided, and wherein said message server is adapted to maintain a record of a plurality of previous messages intended for a called subscriber and registered to the message server record from a respective calling subscriber. Also, although the term "exchange" is used in this specification as a term having its origin in the field of circuit switched networks, the term "exchange" is also intended to cover corresponding devices known as "routers" in the field of packet switched networks and in the filed of "Voice over IP calls" and/or Internet Telephony. Also, the term "call" as used herein is intended to be understood in its broadest sense such that it covers any kind of connection (circuit switched, packet switched, or IP (Internet Protocol) based connections as well as real time, non-real time, constant bit rate or variable bit rate connections) . In this connection, it is to be understood that the present invention is not limited to any particular type of telecommunication system, and the present invention may be applied to a PSTN telephone system such as the ISDN network, or to a mobile radio telecommunication system such as the GSM telecommunication system or to IP telephony systems. Still further, the principles as set out in this specification may also be applied to a telecommunication system still under development such as the UMTS system (3 rd generation mobile telecommunication system) as long as any of such aforementioned systems fulfills the above noted prerequisites of an "exchange" and a "message server". In addition, a message server means any means and/or network element adapted to keep a record of a plurality of previous messages intended for a called subscriber (and/or a plurality of subscribers registered to the network) and registered to (i.e. stored in or recorded to) the message server record from a respective calling subscriber. In this connection, the expression "message" has to be understood in its broadest sense and it may comprise voice mail messages (recorded spoken speech messages) as well as text messages such as SMS messages and any other type of messages such as data messages comprising images and/or text and/or voice (for example also referred to as "multimedia messages") in whatever combination as long as the message has a content which is defined by the calling subscriber . Also, the term "call drop-back feature" as used herein means a feature (service) offered by the telecommunication network which enables a called subscriber listening to the messages registered for him to make a direct call to a respective calling subscriber who has left the listened message for the called subscriber. According to the prior art, several related services in connection with such a service also named call return service have been described. The U.S. patent number US-A- , 930, 152 by Miller (assigned to Bell Communications Research), of May 29, 1990, discloses a call return telephone service. According to the disclosed arrangement, only a list of calling subscribers' telephone numbers is kept in the switch (exchange) of the network for the purpose of calling back the previously calling subscribers. However, this arrangement does not disclose that any specific messages having a content defined by the calling subscriber can be left for the called subscriber. Moreover, as a result, also no message server is provided according to this prior art arrangement. The European patent application number EP-A2-0 795 992 by Hanson & Rudrapatna (assigned to AT&T Corporation) discloses a method and apparatus for a prepaid return call in connection with a message server. According to this prior art arrangement, the called subscriber as the recipient of messages stored on the message server is not enabled to initiate a listening operation for learning the contents of the messages stored for him. Instead, according to this prior art arrangement, the called subscriber is contacted upon initiation of the message server, which periodically tries to contact the called subscriber m order to deliver the message. However, due to periodically contacting the called subscriber by the message server and only a maximum number of such "tries" being permitted, there exists the risk that the message left for the called subscriber may never be delivered to him. Also, as the called subscriber can not influence when the left messages are delivered to him, he may feel uncomfortable if the messages are delivered at an inappropriate time of the day or night. Only if a message could be delivered by the message server to the called subscriber successfully, the called subscriber is offered a possibility to initiate a callback to the calling subscriber (who has prepaid this callback call). Still further, the European patent application EP-A2-0 897 239 by Roth et al. (assigned to Siemens AG) discloses a program controlled communication system having access to a voice mail server. According to his prior art, a called subscriber can retrieve messages intended for him from a voice mail server VMS. A calling subscriber may automatically be called back upon retrieval of the corresponding message from the VMS. However, such a callback mechanism does not allow a direct callback from the ca_ led to the (previously) calling subscriber, since in connection with the callback service, only an exchange of another stored message (from the called subscriber to the previously calling) subscriber is enabled. Stated in other words, only pre-stored messages are exchanged with an interaction of the message server being involved between the called and the calling subscriber or vice versa. Thus, prior art systems known in connection with call back services suffer from a variety of drawbacks and inconveniences for the users (subscribers). SUMMARY OF THE INVENTION Consequently, it is an object of the present invention to enable a called subscriber A to make a callback call to several different calling subscribers during a single message server session. According to the present invention, this object is achieved by a method for performing a call drop-back in a telecommunication system, said telecommunication system comprising at least at least one exchange for switching calls between at least two subscribers or between one of said subscribers and a message server, wherein said message server is adapted to maintain a record of a plurality of previous messages intended for a called subscriber and which previous messages were recorded to the message server record from a respective calling subscriber, wherein said method comprises the steps of: invoking a callback call from said called subscriber to said calling subscriber based on the information recorded in said message server record via at least one of said at least one exchange, and upon invoking said callback call, setting the system into a state enabling said called subscriber to be connected back to said message server after the termination of said invoked callback call. Advantageous further developments of the method according to the present invention are set out in the dependent claims . Still further, the object underlying the present invention is solved by a control device for performing a call drop- back in a telecommunication system, said control device being adapted to carry out the method according to the present invention. Such a control device may even be distributed over at least two telecommunication network elements . Also the present invention relates to a telecommunication network element comprising a control device as aforementioned. Stated in other words, according to the present invention, when a called subscriber A makes a listening call to a message server it is possible to invoke the call drop-back feature (i.e. to make a callback call to the subscriber who has left a message (to which message subscriber A listens when making the listening call) to the message server. The callback call will be dropped to some or at least one of the previous exchanges (or even to the originating exchange) from where the call will be routed to the previously calling subscriber C. The connection between the exchange and the message server will be released immediately and if the re-routed call (callback call) does not succeed or is terminated, the exchange-to-message- server connection (MSC-VMS/SN) will be re-established. When the call / connection between called subscriber A and previously calling subscriber C will be released, it is enabled that the connection between subscriber A and the message server is re-established and that the subscriber A may listen to a next message which has been left at the message server and to make a new callback call to a person (subscriber) who has left that message. In order to accomplish this, needed information will be transmitted between the message server and the exchange in a new parameter, the transmission being based on the usage of ISUP (ISDN user part) signaling. The name of the new parameter also referred to as transmitted "indication" may be "VMS service indicators!'. The parameter / indication includes at least a call drop back type indicator (CDB TYPE INDICATOR) , indicating if a re-establishment of the listening call to the message server (VMS/SN) is needed after the callback call is terminated, a call drop back release reason indicator (CDB RELEASE REASON INDICATOR) , indicating whether the callback call to said calling subscriber (C) has been successful or not, and a message server session indicator (VMS/SN SESSION ID) , containing a subscriber identification of the listening called subscriber A and a message identification identifying the message listened to, these identifications enabling to resume the listening call after the callback call has terminated. Accordingly, with the present invention as briefly outlined above, it becomes advantageously possible that a called subscriber, listening to his messages left for him at the message server, is enabled to carry on listening to his messages after a callback call to the subscriber who has left that message, without the need to make a new call to the message server afterwards. Thus, a called subscriber can make different calls to several different numbers during one message server session. Also, the drawbacks mentioned earlier herein before as attributed to the prior art arrangements are removed by virtue of the present invention being implemented. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more readily understood upon referring to the following description of preferred embodiments when read in conjunction with the accompanying drawings. In the drawings, Fig. 1 shows a schematic block diagram of a telecommunication network architecture and the relevant components required for the present invention to be carried out; Fig. 2 (composed of Figs. 2A and 2B) illustrates a schematic signaling scenario of signals relevant for the present invention which are transmitted between subscribers and telecommunication network elements depicted in Fig. 1 ; Fig. 3 shows a basic example of a message record provided m the message server and the data stored therein; Fig. 4 shows an example of a data format usable for transmission of the VMS service indication; and Fig. 5 shows an actual format of the VMS service indication . DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Subsequently, the present invention will be described in detail with reference to the drawings. Fig. 1 shows a schematic block diagram of a telecommunication network architecture and the relevant components required for the present invention to be carried out. For the specific example described, the architecture of a mobile telecommunication network has been chosen, such as for example, the GSM network. However, also other network architectures may be applied, as mentioned above. Namely, a similar architecture may be applied m connection with packet switched networks and/or Voice over IP networks (Internet Telephony Networks), with the exchange being replaced by a router. Nevertheless, for the purpose of the present invention, the term exchange is intended to cover both meanings, an exchange as such as well as a router. In order to simplify the explanation, the following description focuses on the GSM network architecture as an example . A calling subscriber C, when calling a called subscriber A, communicates via an air interface AIR-I/F with a base station subsystem BSS, which in turn communicates via an interface A-I/F with a network switching subsystem NSS. The base station subsystem BSS comprises at least one base station (not shown) and at least one base station controller (not shown) . The base station subsystem and/or the at least one base station controller (not shown) communicates via the interface A-I/F with a network switching subsystem NSS, particularly, with one of at least one exchange such as a mobile service switching center MSC. Of course, in a network covering a big area and /or a great number of subscribers, plural exchanges, 1. e. MSC's are required and provided for. However, for the sake of keeping the description simple and understandable, the present invention is described with reference to only one exchange MSC. The exchange MSC communicates via an interface VMS-I/F with a message server such as, for example, a voice mail server / service node VMS/SN. Thus, when a calling subscriber C calls a called subscriber A, the signal flow can be depicted as C -> BSS -> MSC -> BSS -> A, in case that subscriber A can be reached and answers his phone. In contrast thereto, in case that the called subscriber A does not answer his phone / terminal either to some defect, due to being busy with another call, or because he has switched off his terminal, this may be informed to the subscriber C. The subscriber C may then initiate to leave a message via the exchange MSC at the message server VMS/SN which is intended for the called subscriber A. The signal flow in this case can be summarized as C -> BSS -> MSC -> VMS/SN. Alternatively, the leaving of a message may be implemented to be performed automatically at the side of the exchange MSC upon detection that the called subscriber is not reachable. At an arbitrary point of time later, the previously called subscriber may then interrogate the message server VMS/SN to learn whether any message has been left for him. In this case, the information flow is A -> BSS -> MSC -> VMS/SN -> MSC -> BSS -> A. The subscriber A thus listening to his voice mail may initiate a direct callback call to - li the calling subscriber who has left the message listened to, without the need for dialing a number. Namely, when A wants to call back C, the call is "dropped back" from the message server to a previous exchange MSC involved in connecting the subscriber A to the message server, from where the callback call is then connected further to the previously calling subscriber C. Previously, a connection between the exchange MSC and the message server VMS/SN was released as soon as the call dropped back to the MSC, so that a new connection to the message server had to be established by the subscriber A after such a callback call, if the subscriber A wanted to continue to listen to his voice mails. According to the present invention and to the new signaling between the exchange and the message server as presented m the present specification, it is enabled to a previously called subscriber listening to his messages such as voice mails, to continue to listen thereto after having made a callback to a calling subscriber without the need to establish a new call to the message server. This will be set out in still greater detail in connection with Figs. 2 to 4. Fig. 2 (composed of Figs. 2A and 2B) illustrates a schematic signaling scenario of signals relevant for the present invention which are transmitted between subscribers and telecommunication network elements depicted in Fig. 1. In order to keep the Figure simple and to simplify the description, the base station subsystem BSS indicated in Fig. 1 has now been omitted m the drawing. Nevertheless, actually, it is still present and involved in the "real" signaling. Moreover, in Fig. 2, the network elements and subscribers involved in the signaling as well as the signaling itself are illustrated in the horizontal direction, while the time axis is intended to be represented m vertical direction. In Fig. 2A, a calling subscriber C, m step SI tries to call a called subscriber A via the exchange MSC (and the base station subsystem BSS (not shown) . In a step Sla, it is detected that the called subscriber A is not reachable. In response thereto, this is either signaled to the subscriber C in order to decide whether he wants to leave a message for the subscriber A (not shown) , or at the side of the exchange MSC there is automatically provided the possibility to the calling subscriber C to leave a message for the called subscriber at the message server VMS/SN, as shown in step S2. Since the way of leaving a message in the message server is not within the scope of the present invention, the corresponding description is somewhat abbreviated and kept short by merely outlining the signal flow in principle. At an arbitrary point of time later, and upon initiation of the previously called subscriber A, in step S3, the subscriber A initiates a listening call via the exchange MSC to the message server VMS/SN in order to listen to one or more messages which have been left for him at the message server VMS/SN. In this way, a VMS/SN session is opened, and the call drop back feature (CDB feature) is invoked. (Note that the listening is assumed to be started from a message number 1 onwards, one message after the other, as will be set out later in greater detail) . Then, in a following step S4, it is detected whether any message (s) is (are) recorded for the listening subscriber in a record at the message server VMS/SN. Although the detection is illustrated as being performed at the message server side, this can also be performed at the exchange side. In the former case, a corresponding control device would then be distributed over a plurality of at least two network elements, while in the latter case, the control device would be located at and/or form part of the exchange MSC. Then , in a following step S5, the listening subscriber A obtains the message information of the stored message (s) . Stated in other words, the message information of one such message (if any) is forwarded to the called subscriber A. The message contains for example a voice mail message together with an identification of the previously calling subscriber C, for example his telephone number MSISDN, or already his name. Having received this information, i.e. having listened to the voice mail, the called subscriber A may personally decide (e.g. dependent on the contents of the listened voice mail and/or dependent on the identity of the subscriber C who has left this message for the called subscriber A) whether he wants to call back the previously calling subscriber C. If so, in step S6, the called subscriber invokes a callback call, the callback call invocation being forwarded via the exchange MSC to the message server VMS/SN. At the message server side, a pointer points to the entry in the record of recorded messages which has immediately been listened to by the called subscriber A. If a callback call is thus invoked, at the message server side an indication, i.e. a VMS service indication is generated and transmitted, step S7, via the VMS-I/F interface to the exchange MSC. This indication, i.e. the VMS service indication is, for example, transmitted using ISUP (ISDN User Part) signaling. Upon having received the transmitted indication, at the exchange side, the listening call is released. More precisely, from step S3 onwards, there exists a connection from subscriber A to the exchange MSC, and also further from the MSC to the message server VMS/SN. Releasing the listening call now means, that only the connection between the exchange MSC to the message server is released. Then, in a step S9 (step S9a and S9b) based on the (still) existing connection between the subscriber A and the exchange MSC (step S9a) , a connection to the previously calling subscriber C is established (step S9b) . This connection between subscribers A and C is also referred to as callback call. Thus, m case the subscriber C can be reached, the subscriber A can directly call back the subscriber C and talk to him. Having finished the conversation between subscribers A and C, the callback call is released, step S10. However, there are other reasons for releasing the callback call. For example, if the callback call can not be established due to the subscriber C being not reachable (e.g. because of being busy in another call, etc., as mentioned above) , also in this case the callback call (attempt) is released and the release reason is returned in step S10 to the exchange MSC. In step Sll, at the exchange side, the transmitted release reason is set and/or written in a call drop-back release reason indicator (CDB release reason indicator) m the VMS service indication (to be described later in greater detail). Also, the VMS session indicator will be written to the VMS service indication parameter. Thereafter, in step S12, the message server session (VMS/SN session) is resumed (if the CDB type indicator in the VMS service indication previously transmitted in step S7 was set to a value indicating indicated "YES"), and m step S13, the record of messages kept at the message server is updated. That is, the CDB release reason transmitted in the indication in step S12 is written into a corresponding message record. In step S14, it is detected whether there is/are one or more other messages recorded at the message server record within the partition assigned to the called /listening subscriber which are not labeled as having served as a basis for a successful callback call to the calling subscriber who has originated the call/ left the message at the message server VMS/SN. If such a message is present, the sequence returns to step S5 and the steps from S5 through S14 are repeated. Otherwise, the message server session is terminated. Thus, the called subscriber A can make a call to several different numbers of previously calling subscribers who have called him, during one message server VMS/SN session. There is no longer a need to make a new call to a message server after a callback call has been made to a previously calling subscriber. The data exchanged between the message server VMS/SN and the exchange MSC form one aspect of the present invention. Therefor, to enhance still further the understanding of the present invention, those data and an example of a possible data format are described with reference to Figs. 3 and 4. Fig. 3 shows an example of a message record MESSAGE RECORD provided in the message server VMS/SN and the data stored therein . The entire message record is partitioned in subscriber partitions. In Fig. 3, a partition for subscriber A is shown. A partition may be identified by the subscribers telephone number MSISDN (e.g. MSISDN A). In a more secure manner, it may be identified by a personal identification code such as a PIN number (personal identification number) and or a combination thereof, also in a ciphered form, if desired. Within each such partition, the following data are kept in respective data fields: 1.) a call number (CALL No.): This indicates the number of a call. In a most simple implementation, the call number is associated with the the time the message has been recorded in the record. For example, call number #1 represents the oldest message, while call number #N represents the most recently recorded message . Alternatively, the order of the messages can be arranged such that they are arranged in an order of priority of the registered message. For example, there may be provided a possibility for a calling subscriber to indicate that his message is urgent (high priority) , of normal importance (medium priority) or of subordinated importance (low priority). Then, the messages can be sorted according to the indicated priority thereof, if necessary in combination with the time of being recorded. 2.) calling subscriber identity (CALLING SUBSCR. ID (MSISDN) No.) This indicates the identity of the calling subscriber, for example by his telephone number (Cl, C2, ..., Cn) , which is used for establishing the callback call. Also, the MSISDN number can be stored together with the subscriber's name in order to simplify the recognition of the calling subscriber to the called subscriber. Also, the messages can be sorted according to the subscribers' telephone numbers and/or names, also in combination with the above mentioned sorting according to time of registration and/or priority. 3.) message (MESSAGE) This indicates a memory area of the message record allotted for storing the message left by the calling subscriber C for the called subscriber A. The message may for example be a voice mail message (VM1, VM2, ..., VMn) , or any other message as mentioned earlier above. . ) call drop back release reason (CDB RELEASE REASON INDICATION) For this entry, at least one indication indicating whether a callback call has been established and performed with success is to be provided. That is, a flag indicating whether subscriber A, upon listening to his voice mail from subscriber C, was successful in calling back subscriber C, is to be provided in a minimum solution. Therefore, in a most general implementation, one bit is already sufficient for encoding this information (e.g. "YES" = 1, representing a normal termination after successful callback, "NO" = 0 representing all other error reasons for non-successful callback) . However, in more sophisticated implementations, depending on the number of reasons, a higher number of bits has to be provided for coding this information. Generally, in order to code 2 n reasons, n bits have to be provided for coding the CDB REELEASE REASON. The data entries in a subscriber partition mentioned under items 1 to 3 herein above are written at the time of recording the message by the calling subscriber C who can not reach the called subscriber A at that time, while the data entry mentioned under item 4 is written in step S13 in Fig. 2B, when the message record is updated upon a callback call establishment. Also, in an alternative implementation, an entry such as "CDB RELEASE" = "YES" (flag being set to "1") need not be written to the record, but a corresponding message could be deleted from the respective subscriber partition in the message record at the message server, thereby continuously decreasing the number of messages in the record after having performed a callback call successfully. Fig. 4 shows a basic example of a data format usable for transmission of the VMS service indication. As illustrated, the VMS service indication comprises three fields, which are partly again divided in sub-fields. A) VMS/SN SESSION ID The VMS/SN SESSION ID identifies the session at the message server to which -after a callback call has been performed- is to be returned. To this end, the VMS/SN SESSION ID field includes a subscriber ID field, e.g. including the subscriber's telephone number MSISDN, such as the MSISDN A. This subscriber ID represents a pointer to the subscriber partition in the message record. Additionally, the VMS/SN SESSION ID includes a message ID, i.e. an identification of the message that has been listened immediately before. For example, the message ID is represented by the call number (mentioned m connection with the Fig. 3 description) . The message ID thus represents a pointer to a specific message having this number within the respective subscriber partition (pointed to by the subscriber ID) . B) CDB RELEASE REASON INDICATION This field is similar and/or identical to the one discussed above in connection with Fig. 3 in so far as its contents is concerned. However, it should be noted that in step S7, this field CDB RELEASE REASON INDICATION of the VMS SERVICE INDICATION does not contain a data entry, or at least one which is commonly known as "don't care" and which has no meaning. In Fig. 5 to be described below, such a data entry is referred to as "No Indication" C) CDB TYPE INDICATOR This call drop-back type indicator indicates whether a return to the message server VMS/SN is necessary and/or possible after a callback call has been established and thereupon released. Thus, also the CDB TYPE INDICATOR may be coded, in a most general implementation, by using one bit indicating "return: yes" (e.g. "1") or "return: no" (e.g. " 0 " ) . Nevertheless, in a practical implementation, the CDB TYPE INDICATOR may assume four values (coded by 2 bits) referred to as "NO indication", "Unsupervised CDB", "Supervised CDB" or "CDB return" (the latter being also referred to as "intelligent CDB") . Upon receipt of the corresponding information, the exchange MSC then initiates an appropriate action depending on the received information. Further details are set out in connection with the description of Fig. 5. The setting of the CDB TYPE INDICATOR in connection with the above practical implementation may already be performed at the time of manufacturing when implementing the method feature in the network device (s). Nevertheless, it can also be provided for that the CDB TYPE INDICATOR will be set in the VMS/SN according to the instructions of the subscriber A listening to the messages left for him at the VMS/SN when the subscriber A configures his services. However, it is also conceivable to adopt a kind of "dynamic" setting of the CDB TYPE INDICATOR, which may then be performed at the message server side, prior to step S7. Namely, the VMS/SN may be adapted to make the decision concerning the CDB TYPE INDICATORE depending an the services offered by the VMS/SN to the subscriber A and depending on whether there are messages left in the message server VMS/SN. In this connection, for example, the setting of the CDB TYPE INDICATOR may be performed based on the following examples of a logic evaluation of data items described before : CDB TYPE INDICATOR = "YES" if current call number < maximum call number n OR "YES" if any CDB RELEASE REASON INDICATION in the record (of the partition) indicates "not successful". (In the latter case ("the OR clause"), a retry for a callback call for a previously unsuccessful callback call attempt can result in the flow) . CDB TYPE INDICATOR = "NO" if current call number = maximum call number AND no CDB RELEASE REASON INDICATION in the record of the partition indicates "not successful". Of course, various modifications of the above mentioned logic evaluations are further conceivable. Also, the sequence of indicators within the indication is not limited to the one presented and another data arrangement may be adopted. Also, in step S12, the CDB TYPE INDICATOR need not be transmitted back to the message server, as it has been evaluated before, so that in such a case, also the value "NO indication" may be transmitted. Fig. 5 shows an actual format of the VMS service indication which shows the subfields of the VMS service indication in greater detail. As shown in Fig. 5, the VMS service indication comprises spare fields and /or spare bits which are used to provide for a possibility to expand the indicators in the future, if necessary. For example, it could become necessary to code more CDB release reasons etc., for which coding more bits are required. The CDB type indicator is composed of two bits, thus enabling the coding of four contents and/or meanings, namely: "00" representing "No Indication"; the value "No indication" is used in the CDB type indicator in the IAM (Initial Address Message) with which the MSC-VMS/SN connection will be re-established (see step S12 m Fig. 2B) . If there is the value "No indication" in the REL (RELease) message which invokes the call dropback functionality, the exchange MSC is caused to operate like in case of the Unsupervised CDB (see below), where there is no return to the VMS/SN connection. "01" representing an "Unsupervised CDB", according to which setting the call does not return to the VMS/SN in any case after a callback call has been initiated to the (previously) calling subscriber C. This is an option for a setting m case a network operator does not wish to have the present invention being implemented in his network, e.g. for cost reasons, "10" representing a "Supervised CDB", according to which setting the call is routed back to the VMS/SN after a callback call to the calling subscriber C has not been successful (e.g. if the CDB release reason does not indicate a normal termination of the CDB call), and "11" representing an "Intelligent CDB", according to which setting the call returns to the VMS/SN if the callback call to the calling subscriber has not succeeded and if the callback call to the calling subscriber C has succeeded . Furthermore, the CDB release reason is coded using three bits, thus enabling the coding of up to eight CDB release reasons. Five values are used in connection with the illustrated actual format of the VMS service indication, namely : "000" representing "NO Indication" (transmitted for example in step S7, when a CDB release reason at the attempt of a callback call is yet not determined) , "001" representing that the previously calling party C (no being called back) is not reachable, e.g. due to the terminal being switched off, "010" representing that the calling party C being busy, i.e. occupied by another call and/or data transmission, "100" representing a normal termination of the CDB, i.e. that the callback call has been established successful and that the call has been normally terminated, e.g., after the subscribers A and C have talked to each other, and "111" representing that subscriber C does not reply to the callback call attempt although he is reachable while not being busy. Finally, the VMS Session Indicator consists of 8 to 28 bytes having a contents as discussed above, i.e. containing the subscriber ID and a message ID needed to resume the VMS/SN session after the (new) callback call has finished or after an unsuccessful establishment of the callback call. Herein above, the present invention has mainly been described with reference to a method. Nevertheless, the present invention also aims to a corresponding control device adapted to carry out the method. To this end, a control device comprises (not shown) accordingly adapted means for listening / session opening / CDB feature invocation, detection, obtaining, callback call invocation, indication transmission, listening call release, callback call establishment and release, setting session resumption and indication transmission, as well as for updating. Also, those means may be provided in a single network element such as the exchange (MSC) , but where appropriate, the means constituting the control device may also be distributed over at least two telecommunication network elements e.g. over the exchange MSC and the message server VMS/SN. Accordingly, as has been described herein above, the present invention proposes a method for performing a call drop-back in a telecommunication system, said telecommunication system comprising at least at least one exchange (MSC) for switching calls between at least two subscribers (A, C) or between one of said subscribers and a message server (VMS/SN) , wherein said message server (VMS/SN) is adapted to maintain a record of a plurality of previous messages intended for a called subscriber (A) and which previous messages were recorded to the message server record from a respective calling subscriber (C) , wherein said method comprises the steps of: invoking (Sβ) a callback call from said called subscriber (A) to said calling subscriber (C) based on the information recorded in said message server record via at least one of said at least one exchange (MSC) , and upon invoking said callback call, setting the system into a state enabling said called subscriber (A) to be connected back to said message server (VMS/SN) after the termination of said invoked callback call. Also, a corresponding control device as well as a network element comprising the same are proposed. It should be understood that the above description and accompanying figures are merely intended to illustrate the present invention by way of example only. The preferred embodiments of the present invention may thus vary within the scope of the attached claims.

Claims: CLAIMS 1. A method for performing a call drop-back in a telecommunication system, said telecommunication system comprising at least at least one exchange (MSC) for switching calls between at least two subscribers (A, C) or between one of said subscribers and a message server (VMS/SN) , wherein said message server (VMS/SN) is adapted to maintain a record of a plurality of previous messages intended for a called subscriber (A) and which previous messages were recorded to the message server record from a respective calling subscriber (C) , wherein said method comprises the steps of: invoking (S6) a callback call from said called subscriber (A) to said calling subscriber (C) based on the information recorded in said message server record via at least one of said at least one exchange (MSC) , and upon invoking said callback call, setting the system into a state enabling said called subscriber (A) to be connected back to said message server (VMS/SN) after the termination of said invoked callback call. 2. A method according to claim 1, wherein said setting step includes a step of transmitting (S7) an indication of a necessity for re-routing a call from said subscriber (A) to said message server (VMS/SN) after said callback call from said called subscriber (A) to said calling subscriber (C) will be terminated, said indication being transmitted from said message server (VMS/SN) to said at least one of said at least one exchange (MSC) via a corresponding interface (VMS-I/F) . 3. A method according to claim 1, comprising, before the step of invoking, the steps of establishing (S3) a listening call from a previously called (SI) subscriber (A) via at least one of said at least one exchange (MSC) to said message server (VMS/SN) , and detecting (S4) that a calling subscriber (C) has previously recorded a message for said called subscriber (A) in said message server record (VMS/SN) . 4. A method according to claim 3, comprising, after the step of transmitting, the steps of releasing (S8) said listening call and establishing (S9, S9a, S9b) a callback call from said called subscriber (A) via at least one of said at least one exchange (MSC) to said calling subscriber (C) , and upon terminating said established callback call, re-establishing (S12) the listening call from said subscriber (A) via said at least one of said at least one exchange (MSC) to said message server (VMS/SN) , based on said transmitted indication. 5. A method according to claim 4, wherein prior to re-establishing said listening call, upon terminating said established callback call, a call drop back release reason indicator, indicating whether the callback call to said calling subscriber (C) has been successful or not, is written (Sll) in said indication. 6. A method according to claim 2, wherein said transmitted indication is transmitted via said interface (VMS-I/F) using an ISDN User Part signaling (ISUP). 7. A method according to claim 2 or 4, wherein said indication includes at least a call drop back type indicator, indicating if a re- establishment of the listening call to the message server (VMS/SN) is needed after the callback call is terminated, a call drop back release reason indicator, indicating whether the callback call to said calling subscriber (C) has been successful or not, and a message server session indicator, containing a subscriber identification of the listening called subscriber (A) and a message identification, these identifications enabling to resume the listening call after the callback call has terminated. 8. A method according to claim 5, further comprising the steps of, after writing said release reason indicator into said indication, transmitting (S12) said indication from said exchange (MSC) to said message server (VMS/SN) , and updating (S13) said message record at the message server using information contained in said indication. 9. A control device for performing a call drop-back in a telecommunication system, said control device being adapted to carry out the method according to any of the foregoing claims 1 to 8. 10. A control device according to claim 9, wherein said control device is distributed over at least two telecommunication network elements (MSC, VMS/SN) . 11. A telecommunication network element comprising a control device according to claim 9.

Inventors: ASKO KOIVULA, KALLIO SEPPO, KLINT JUHANI, LAMPINEN JUHA, LUOTO TERO, MAEKINEN ILKKA, MATTSSON PATRIK, TAPONEN MATTI, VALLINEN JUHA, ASKO, KOIVULA, KALLIO, SEPPO, KLINT, JUHANI, LAMPINEN, JUHA, LUOTO, TERO, MAEKINEN, ILKKA, MATTSSON, PATRIK, TAPONEN, MATTI, VALLINEN, JUHA
Applicants: ASKO KOIVULA, KALLIO SEPPO, KLINT JUHANI, LAMPINEN JUHA, LUOTO TERO, MAEKINEN ILKKA, MATTSSON PATRIK, NOKIA NETWORKS OY, TAPONEN MATTI, VALLINEN JUHA, ASKO, KOIVULA, KALLIO, SEPPO, KLINT, JUHANI, LAMPINEN, JUHA, LUOTO, TERO, MAEKINEN, ILKKA, MATTSSON, PATRIK, TAPONEN, MATTI, VALLINEN, JUHA
Application year: 1999
Main classification: H04M3
IPC codes: H04M3