Abstract:
The present invention faces the issue of exercising a parental control over children&#39;s activities in the IMS network, in terms of registering into the IMS network as well as in terms of call establishment, and provides for a new IMS subscription model supporting a hierarchy of IMPI&#39;s, so-called ‘primary’ IMPI&#39;s and so-called ‘secondary’ IMPI&#39;s, whereby only the primary IMPI&#39;s are allowed to register themselves, whereas the secondary IMPI&#39;s can only register themselves after having been previously registered by a primary IMPI. To this end, the present invention provides for a new method and an enhanced HSS to allow the implicit registration of one or more ‘secondary’ IMPI&#39;s upon the explicit registration of an IMS subscriber with a given IMPI/IMPU pair, wherein the given IMPI is a ‘primary’ IMPI and the given IMPU is associated with a registration set of IMPI&#39;s which includes said one or more ‘secondary’ IMPI&#39;s.

Description:
TECHNICAL FIELD 
     The present invention relates to parental control for accessing an IP Multimedia Subsystem network. More particularly, the invention pertains to means and method for an implicit registration of a number of Private User Identities of a subscriber of an IP Multimedia Subsystem upon an explicit registration of the subscriber in the IP Multimedia Subsystem with a given Private User Identity and a given Public User Identity. 
     BACKGROUND 
     3GPP defines an IP Multimedia Subsystem and, more specifically, the IP Multimedia Core Network Subsystem to enable support for IP multimedia applications. For the sake of simplicity, and since the IP Multimedia Subsystem is defined by 3GPP as being access-independent, all references are made throughout this specification to the IP Multimedia Subsystem (hereinafter IMS). 
     According to 3GPP, a user may register into the IMS network or receive a terminating call to experience IMS services. To this end, such user must be given a subscription to the IMS. 
     A user with an IMS subscription is given one or more Private User Identities. An IMS Private User Identity (hereinafter IMPI) is assigned by the home network operator, and is used for Registration, that is, for Authorization and Authentication into the IMS network. A user may also have one or more Public User Identities. An IMS Public User identity (hereinafter IMPU) is used by any user as a user&#39;s identifier for communications with other users. 
     Generally speaking, any IMPU of an IMS subscription may be shared by more than one IMPI within the same IMS subscription. In particular, an IMPU may be shared amongst all the IMPI&#39;s of an IMS subscription as stated in 3GPP TS 23.228. This feature is called IMPU sharing, and such IMPU is generally known under 3GPP as a ‘shared IMPU’. 
     In this respect,  FIG. 1  illustrates an exemplary IMS subscription in accordance with 3GPP, wherein “Public User Identity—3” and “Public User Identity—4” are both shared by all the IMPI&#39;s of the IMS subscription, namely “Private User Identity—1” and “Private User Identity—2”, and are thus both considered ‘shared IMPU&#39;s’. 
     On the one hand, an IMS subscriber may register into the IMS network with an IMPI/IMPU pair selected by the IMS subscriber amongst those IMPI&#39;s and IMPU&#39;s associated in the IMS subscription of the IMS subscriber. The IMS subscriber thus registers into the IMS with a ‘Register’ message of a Session Initiation Protocol (hereinafter SIP), as defined by 3GPP, and including a selected IMPU/IMPI pair. Moreover, 3GPP further discloses a so-called ‘implicit registration set’ (hereinafter IRS) of more than one IMPU so that, where a given IMPU registered in an IMPI/IMPU pair is included in an IRS, all the IMPU&#39;s included in said IRS are considered to be registered as well. 
     On the other hand, 3GPP TS 24.229 Rel-7 introduces the concept of contact addresses into the IMS network. In this respect, the contact address can be defined as a SIP Uniform Resource Identifier (hereinafter a ‘SIP URI’) containing the IP address of the user equipment (hereinafter UE). Under certain circumstances, a contact address may also contain an instance identifier that uniquely identifies a specific UE amongst all other UEs registered with a same IMPU. For the sake of simplicity, this contact address may indistinctly be referred to as ‘contact address’ or simply as ‘contact’ throughout this specification. 
     A conventional registration process includes the submission of a ‘SIP Register’ message from the IMS subscriber towards a so-called Proxy Call Session Control Function server (hereinafter P-CSCF), which forwards such message towards an Interrogating Call Session Control Function server (hereinafter I-CSCF) of the IMS network where the destination subscriber belongs to. In particular, this ‘SIP Register’ message includes a given IMPI/IMPU pair to be registered during this registration process, and a contact address associated with the currently used UE. The I-CSCF is in charge of selecting an appropriate Serving Call Session Control Function server (hereinafter S-CSCF) for serving the IMS subscriber, and queries a Home Subscriber Server (hereinafter HSS), which is in charge of subscription data for subscribers of the IMS network where the IMS subscriber belongs to, with the given IMPI/IMPU pair. Assuming that the IMS subscriber had not previously registered the IMPI/IMPU pair, the HSS returns the capabilities required for an S-CSCF to be assigned for serving the IMS subscriber. The I-CSCF receiving such capabilities selects an appropriate S-CSCF fulfilling the capabilities, and forwards the ‘SIP Register’ message with the IMPI/IMPU pair and the contact address thereto. The S-CSCF receiving the ‘SIP Register’ message submits its own registration towards the HSS to indicate it has been assigned for serving the subscriber identified by the IMPI/IMPU pair. The HSS then changes the status of said IMPI and IMPU from ‘not registered’ to ‘registered’, it stores a reference to the S-CSCF as being assigned for serving the IMS subscriber, and it downloads a user profile associated with said IMPU towards the S-CSCF. The S-CSCF receiving the user profile for the IMS subscriber and already having the given IMPI/IMPU pair and the contact address is now ready for serving the IMS subscriber. 
     In accordance with the current registration mechanism as described in 3GPP, an IMS subscriber is registered in the network with a given IMPU/IMPI pair and with a given contact address. Then, where the IMS subscriber wants to register with another IMPI of the same IMS subscription or with another contact address, the same previous registration mechanism can to be repeated with said another IMPI or another contact address. Regarding the registration of contacts, when the user initiates a new registration attempt, the new contact traverses the network within the SIP header until the ‘SIP Register’ message arrives to the S-CSCF. The S-CSCF stores the contact bound to the IMPU received in the SIP message or to the IRS received from HSS during the registration process. 
     At present, some network operators propose the concept of a so-called ‘familiar subscription’ as an IMS subscription consisting of several IMPI&#39;s identifying the members of the family or even the roles that each member play in the family. For instance, an exemplary ‘familiar subscription’ may consist of a first IMPI identifying the mother, a second IMPI identifying the father, a third IMPI identifying the daughter and a fourth IMPI identifying the son. With this approach, the operators can make special offers to families for contracting an IMS ‘familiar subscription’ and to charge them as a whole rather than independently for every member. 
     Nowadays, there are quite a few families with kids and teenagers, whose parents would like to have a control of the time their children spend with IMS services. 
     However, with the currently existing mechanisms for accessing the IMS network and for IMS session establishment, either terminating or originating session, there is no mechanism to exercise a parental control over children&#39;s activities in the IMS network. 
     SUMMARY 
     The present invention is aimed to at least minimize the above drawback and provides for a new IMS subscription model supporting a hierarchy of IMPI&#39;s, the so-called ‘primary’ IMPI&#39;s and the so-called ‘secondary’ IMPI&#39;s, whereby only the primary IMPI&#39;s are allowed to register themselves on their own, whereas the secondary IMPI&#39;s are not allowed to register themselves unless they have been previously registered by any primary IMPI. Therefore, a new method and an enhanced HSS are provided to allow the implicit registration of one or more secondary IMPI&#39;s upon the explicit registration of an IMS subscriber with a given IMPI/IMPU pair, wherein the given IMPI is a primary IMPI and the given IMPU is associated with a registration set of IMPI&#39;s which includes said one or more secondary IMPI&#39;s. 
     In accordance with a first aspect of the present invention, there is provided a new method of registering, during a single registration process in an IP Multimedia Subsystem “IMS”, an implicit registration set of ‘j’ IMPI&#39;s. 
     This method comprises the steps of: configuring at a HSS an IMS subscription for a subscriber with a number ‘n’ of IMPI&#39;s and a number ‘m’ of IMPU&#39;s, wherein each IMPI is associated with at least one IMPU and each IMPU is associated with at least one IMPI, and wherein an IMPU is shared by more than one IMPI; configuring at the HSS this IMS subscription with an implicit registration set of ‘j’ IMPI&#39;s associated with the shared IMPU, wherein the ‘j’ IMPI&#39;s are preferably selected amongst the ‘n’ IMPI&#39;s in the IMS subscription; configuring at the HSS at least one IMPI as ‘primary’ IMPI and any other IMPI as ‘secondary’ IMPI of this IMS subscription; receiving at the HSS from a S-CSCF, which is currently assigned for serving the subscriber, an indication of a registration of said subscriber with a given IMPU and a given IMPI, and an identifier of said S-CSCF; determining at the HSS whether the given IMPI is configured as a ‘primary’ IMPI or as a ‘secondary’ IMPI; and either downloading from the HSS towards the S-CSCF the implicit registration set of ‘j’ IMPI&#39;s, where the given IMPI is a ‘primary’ IMPI and the given IMPU is the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s; or rejecting the registration of the subscriber with the given IMPU and given IMPI, where the given IMPI is a ‘secondary’ IMPI not previously registered by a ‘primary’ IMPI within an implicit registration set. 
     Generally speaking for this method, the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s is not necessarily an IMPU shared by all the IMPI&#39;s in the IMS subscription, in accordance with the concept of ‘shared IMPU’ stipulated by 3GPP, but simply an IMPU shared by the given IMPI and those IMPI&#39;s in the implicit registration set of ‘j’ IMPI&#39;s. In particular, the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s may be an IMPU shared by all the IMPI&#39;s in the IMS subscription, as stipulated by 3GPP. 
     An advantageous operation may be achieved, where this method further comprises a step of configuring at the HSS each ‘secondary’ IMPI of the IMS subscription as ‘barred’ for registration. In particular, this step of configuring each ‘secondary’ IMPI as ‘barred’ for registration may include a step of barring for own registration the ‘secondary’ IMPI, which is advantageous on the determination on whether the secondary IMPI may register itself or not. 
     Where the above barring mechanism is implemented, the step of determining in this method that the given IMPI is a ‘primary’ IMPI and the given IMPU is the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s further comprises a step of unbarring for own registration those ‘secondary’ IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s. 
     Additionally, and in order to avoid that a kid previously registered by the parents further registers other kid prevented by the parents from registration, the step of configuring each ‘secondary’ IMPI as ‘barred’ for registration may include a step of barring the ‘secondary’ IMPI for registration of the implicit registration set of ‘j’ IMPI&#39;s associated with the given IMPU. 
     Moreover, apart from barring the own registration of a ‘secondary’ IMPI not previously registered by the parents, the parental control may be advantageously complemented by an effective control over call establishment procedures. To this end, the method may further comprise a step of configuring at the HSS each non-shared IMPU associated with each ‘secondary’ IMPI of the IMS subscription as ‘barred’ for call establishment. 
     Where this barring mechanism is implemented to control the call establishment procedures, the step of determining in this method that the given IMPI is a ‘primary’ IMPI and the given IMPU is the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s further comprises a step of unbarring for call establishment the non-shared IMPU&#39;s associated with each ‘secondary’ IMPI included in the implicit registration set of ‘j’ IMPI&#39;s. 
     An exemplary IMS familiar subscription may include, apart from the children to be carefully controlled by the parents, other family members responsible for their own registration but whom control of children cannot be given. This may be the case of an older brother who needs accessing the IMS services at any time but who should not be fully responsible of younger brothers. To this end, this method may further comprise a step of configuring at the HSS a ‘primary’ IMPI of the IMS subscription, the one for the exemplary older brother, as ‘barred’ for registration of any implicit registration set of ‘j’ IMPI&#39;s associated with a shared IMPU. 
     With this method, parents make sure that young children cannot access the IMS on their own at any time but only during fixed periods under their direct control or supervision, and this access to IMS services terminates for the young children as soon as the parents deregister the implicit registration set of ‘j’ IMPI&#39;s, including those ‘secondary’ IMPI&#39;s assigned to the children. 
     For this purpose, this method may further comprise the steps of: receiving at the HSS an indication of deregistering a subscriber with a given IMPI and a given IMPU from the S-CSCF; determining at the HSS that the given IMPI is a ‘primary’ IMPI and the given IMPU is the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s; and deregistering from the HSS all those ‘secondary’ IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s with any IMPU they had previously been registered. 
     Where the above barring mechanism is implemented to control the own registration, the step of deregistering from the HSS all those ‘secondary’ IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s may be followed by a step of barring for own registration at the HSS those ‘secondary’ IMPI&#39;s (included in the implicit registration set of ‘j’ IMPI&#39;s. 
     Likewise, where the above barring mechanism is implemented to control the call establishment procedures, the step of deregistering from the HSS all those ‘secondary’ IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s may be followed by a step of barring for call establishment the non-shared IMPU&#39;s associated with each ‘secondary’ IMPI included in the implicit registration set of ‘j’ IMPI&#39;s. 
     Even though the step of downloading from the HSS towards the S-CSCF the implicit registration set of IMPI&#39;s associated with the given IMPU may be carried out at any time during or after concluding the registration process, advantages may be obtained in terms of data consistency and simplicity where said step of downloading the implicit registration set is carried out along with the downloading during the registration process, if any, of those IMPU&#39;s in an Implicit Registration Set associated with the given IMPU explicitly registered. 
     The method may be improved with additional steps in order to avoid a terminating call to reach a ‘secondary’ IMPI, which might occur under certain service criteria where said ‘secondary’ IMPI is not registered, since such terminating call may simply address an IMPU associated with the ‘secondary’ IMPI. To this end, this method may further comprise the steps of: receiving a query at the HSS from an I-CSCF, which is in charge of receiving an invitation to communicate with a terminating IMS subscriber, about a subscriber identified by a second given IMPU; determining at the HSS that the second given IMPU is a non-shared IMPU associated with a ‘secondary’ IMPI not previously registered by a ‘primary’ IMPI within an implicit registration set; and rejecting the query about the subscriber with the second given IMPU towards the I-CSCF. 
     In accordance with a second aspect of the present invention, there is provided a new HSS for holding subscriptions for subscribers of the IMS and arranged to configure and download an implicit registration set of ‘j’ IMS private identities. 
     This HSS comprises an accessible storage for configuring an IMS subscription for a subscriber with a number ‘n’ of IMPI&#39;s and a number ‘m’ of IMPU&#39;s, wherein each IMPI is associated with at least one IMPU and each IMPU is associated with at least one IMPI, and wherein an IMPU is shared by more than one IMPI; and wherein this accessible storage is arranged for configuring the IMS subscription for the subscriber with an implicit registration set of ‘j’ IMPI&#39;s associated with the shared IMPU, wherein the ‘j’ IMPI&#39;s are preferably selected amongst the ‘n’ IMPI&#39;s in the IMS subscription, and for configuring at least one IMPI in the IMS subscription as ‘primary’ IMPI and any other IMPI as ‘secondary’ IMPI of the IMS subscription for the subscriber. This HSS also comprises a receiver for receiving from a S-CSCF, which is assigned for serving the subscriber, an indication of a registration of said subscriber with a given IMPI and a given IMPU, and an identifier of said S-CSCF; and a processing unit for determining whether the given IMPU and the given IMPI are associated; wherein this processing unit is arranged for determining whether the given IMPI is a ‘primary’ IMPI or a ‘secondary’ IMPI, whether a ‘secondary’ IMPI has been previously registered within an implicit registration set, and whether the given IMPU is the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s. Moreover, this HSS also comprises a sender for downloading towards the S-CSCF the implicit registration set of ‘j’ IMPI&#39;s, where the given IMPI is a ‘primary’ IMPI and the given IMPU is the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s; or for rejecting the registration of the subscriber with the given IMPU and given IMPI towards the S-CSCF, where the given IMPI is a ‘secondary’ IMPI and the given IMPI has not previously been registered by a ‘primary’ IMPI within an implicit registration set. 
     As for the above method, and in order to benefit from data consistency and simplicity, the sender of the HSS may be arranged for downloading during the registration process the implicit registration set of ‘j’ IMPI&#39;s towards the S-CSCF along with an Implicit Registration Set of IMPU&#39;s, if any, associated with the given IMPU. 
     Aligned with the above method, the accessible storage of this HSS may advantageously include per ‘secondary’ IMPI of the IMS subscription a ‘barring’ indicator configured for barring an own registration of the ‘secondary’ IMPI, and may advantageously include per ‘secondary’ IMPI of the IMS subscription a ‘barring’ indicator configured for barring a registration of an implicit registration set of ‘j’ IMPI&#39;s. Where the accessible storage is arranged for barring an own registration of the ‘secondary’ IMPI, the processing unit may advantageously be arranged for unbarring and barring in the accessible storage the own registration of those ‘secondary’ IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s. 
     Also aligned with the above method, the accessible storage of this HSS may advantageously include per each non-shared IMPU associated with each ‘secondary’ IMPI of the IMS subscription a ‘barring’ indicator configured for barring the non-shared IMPU for call establishment. Where the accessible storage is arranged for barring the non-shared IMPU for call establishment, the processing unit may advantageously be arranged for unbarring and barring in the accessible storage the call establishment for the non-shared IMPU&#39;s associated with each ‘secondary’ IMPI included in the implicit registration set of ‘j’ IMPI&#39;s. 
     In order to terminate the access to IMS services for the young children, the receiver of this HSS may be arranged for receiving from the S-CSCF an indication of deregistering a subscriber with a given IMPI and a given IMPU; and, responsive to this deregistration, the processing unit of the HSS may be arranged for determining that the given IMPI is a ‘primary’ IMPI and the given IMPU is the shared IMPU associated with the implicit registration set of ‘j’ IMPI&#39;s; and, in cooperation with the sender of the HSS, the processing unit may be arranged for deregistering towards the S-CSCF all those ‘secondary’ IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s with any IMPU they had previously been registered. 
     Particularly advantageous for the above method to avoid a terminating call reaching a ‘secondary’ IMPI, the receiver of the HSS may be arranged for receiving a query from an I-CSCF, which is in charge of receiving an invitation to communicate with a terminating IMS subscriber, about a subscriber identified by a second given IMPU; and, responsive to this query, the processing unit of the HSS may be arranged for determining that the second given IMPU is a non-shared IMPU associated with a ‘secondary’ IMPI not previously registered by a ‘primary’ IMPI within an implicit registration set; and the sender of the HSS may be arranged for rejecting the query about the subscriber with the second given IMPU towards the I-CSCF. 
     In accordance with a third aspect of the present invention, there is provided a new S-CSCF for serving subscribers of the IMS. This S-CSCF comprises a sender for submitting towards a HSS, which holds subscriptions for subscribers of the IMS, an indication of a registration of an IMS subscriber with a given IMPU and a given IMPI, and an identifier of the S-CSCF; a receiver for receiving from the HSS an implicit registration set including a number ‘j’ of IMPI&#39;s associated with the given IMPU; a processing unit for determining that the given IMPI is a ‘primary’ IMPI of the IMS subscription; and an accessible storage for storing the ‘primary’ IMPI along with the implicit registration set of ‘j’ IMPI&#39;s, and the given IMPU explicitly registered. 
     In particular, the receiver of this S-CSCF may be arranged for receiving from the HSS an indication per IMPI in the implicit registration set of ‘j’ IMPI&#39;s indicating whether such IMPI is a ‘primary’ or ‘secondary’ IMPI of the IMS subscription. Also in particular, the processing unit may be arranged for determining that the given IMPI is a ‘primary’ IMPI of the IMS subscription in cooperation with the receiver receiving the indication per IMPI on whether such IMPI is a ‘primary’ or ‘secondary’ IMPI of the IMS subscription. 
     Moreover, the receiver of this S-CSCF may be arranged for receiving from the HSS a set with a number ‘k’ of contact addresses to reach those IMPI&#39;s currently registered; and the accessible storage may be arranged for storing the set of ‘k’ contact addresses in association with the IMPI&#39;s currently registered, and with the given IMPU explicitly registered. 
     On the other hand, the invention may be practised by a computer program, in accordance with a fourth aspect of the invention, the computer program being loadable into an internal memory of a computer with input and output units as well as with a processing unit, and comprising executable code adapted to carry out the above method steps. In particular, this executable code may be recorded in a carrier medium readable in the computer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, objects and advantages of the invention will become apparent by reading this description in conjunction with the accompanying drawings, in which: 
         FIG. 1  basically represents a conventional model of an IMS subscription, in respect of user identities and their relationships, as defined by 3GPP. 
         FIG. 2  shows a simplified view of an exemplary IMS familiar subscription to better describe embodiments of the invention. 
         FIG. 3  shows an exemplary configuration at the HSS of ‘primary’ and ‘secondary’ IMPI&#39;s along with respective implicit registration sets of contact addresses and ‘barring’ indicators per IMPI basis. 
         FIG. 4  shows an exemplary configuration at the HSS of implicit registration sets of IMPI&#39;s per IMPU basis along with respective implicit registration sets of IMPU&#39;s and ‘barring’ indicators. 
         FIG. 5  illustrates a simplified view of the sequence of actions to be performed to carry out a method of registering, during a single registration process in an IMS network, an implicit registration set of ‘j’ IMPI&#39;s, in accordance with an aspect of the present invention. 
         FIG. 6  illustrates an exemplary implementation of a HSS provided for configuring and downloading an implicit registration set of ‘j’ IMPI&#39;s, wherein the accessible storage is provided by an internal memory integrated into the HSS. 
         FIG. 7  illustrates an exemplary implementation of a HSS provided for configuring and downloading an implicit registration set of ‘j’ IMPI&#39;s, wherein the accessible storage is provided by an external database acting as an HSS back-end shared by a plurality of HSS front-ends. 
         FIG. 8  shows an exemplary data model of implicit registration sets of IMPI&#39;s and contact addresses built up at the S-CSCF, and downloaded from the HSS, in accordance with the exemplary IMS subscription illustrated in  FIG. 2 . 
         FIG. 9  illustrates an exemplary implementation of an S-CSCF provided for downloading an implicit registration set of ‘j’ IMPI&#39;s, and for building up a data model thereof. 
         FIG. 10  illustrates a simplified view of the sequence of actions to carry out to avoid a terminating call reaching a ‘secondary’ IMPI not previously registered by a ‘primary’ IMPI within an implicit registration set of ‘j’ IMPI&#39;s. 
         FIG. 11  illustrates a simplified view of the sequence of actions to carry out for deregistering all those ‘secondary’ IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s upon deregistration of the ‘primary’ IMPI. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes currently preferred embodiments of means and method for registering an implicit registration of one or more secondary IMPI&#39;s upon the explicit registration of an IMS subscriber with a given IMPI/IMPU pair, wherein the given IMPI is a primary IMPI and the given IMPU is associated with a registration set of IMPI&#39;s which includes said one or more secondary IMPI&#39;s. 
       FIG. 2  illustrates an exemplary IMS familiar subscription  100  for a subscriber of an IMS network, the IMS subscription including a number of IMPI&#39;s  101 - 106  assignable for usage to family members, and another number of IMPU&#39;s  107 - 115  wherein some of them are exclusive of some family member whereas others are shared by several or all family members. In this exemplary IMS subscription  100  the first IMPU  107  is exclusively associated with the first IMPI  101  assigned to the mother; the second IMPU  108  is exclusively associated with the second IMPI  102  assigned to the father; the third IMPU  109  and the fourth IMPU  110  are shared IMPU&#39;s associated with all the IMPI&#39;s of the IMS subscription; the fifth IMPU  111  is a shared IMPU associated with the first IMPI  101  assigned to the mother, with the second IMPI  102  assigned to the father, and with the fourth IMPI  104  assigned to the son; the sixth IMPU  112  is exclusively associated with the fourth IMPI  104  assigned to the son; the seventh IMPU  113  is a shared IMPU associated with the first IMPI  101  assigned to the mother, with the second IMPI  102  assigned to the father, and with the fifth IMPI  105  assigned to the daughter; the eighth IMPU  114  is exclusively associated with the fifth IMPI  105  assigned to the daughter; and the ninth IMPU  115  is exclusively associated with the sixth IMPI  106  assigned to an older son. 
     In accordance with the invention, there is at least one registration set of IMPI&#39;s (hereinafter RSI) associated with a shared IMPU and to be implicitly registered upon registration of a given IMPI/IMPU pair, wherein the given IMPI is configured as a ‘primary’ IMPI associated with the given IMPU and the latter being the shared IMPU associated with the RSI. 
     As illustrated in  FIG. 2  for the exemplary IMS familiar subscription  100 , a first RSI  116  is associated with the shared IMPU  109  and including the first IMPI  101 , second IMPI  102 , fourth IMPI  104  and fifth IMPI  105 . Likewise, a second RSI  117  is associated with the shared IMPU  110  and including the fourth IMPI  104  and fifth IMPI  105 ; a third RSI  118  is associated with the shared IMPU  111  and just including the fourth IMPI  104 ; and a fourth RSI  119  is associated with the shared IMPU  113  and just including the fifth IMPI  105 . 
     Particularly in this exemplary subscription, some shared IMPU&#39;s, namely the third IMPU  109  and the fourth IMPU  110 , are shared IMPU&#39;s in accordance with the stipulations made by 3GPP whereby a shared IMPU is shared by all the IMPI&#39;s in the IMS subscription, whereas other shared IMPU&#39;s, namely the fifth IMPU  111  and the seventh IMPU  113 , are shared IMPU&#39;s in accordance with a less restrictive concept used throughout this specification in the sense that they are shared by more than one IMPI of the IMS subscription. 
     Also in this exemplary IMS subscription, there is provided a third IMPI  103  assignable to the parents, being the mother or the father, associated with the third IMPI  103  and fourth IMPI  104  for the purpose of registering at home any of these shared IMPI  103  and fourth IMPI  104 , and usable to allow the implicit registration of IMPI&#39;s assigned to the younger kids whilst keeping the parents exclusive IMPI&#39;s  101 - 102  in a separate registration status. 
       FIG. 3  illustrates an exemplary configuration  120  at the HSS with configuration elements  121 - 126  per IMPI basis in accordance with the exemplary IMS subscription  100  illustrated in  FIG. 2 . This exemplary configuration  120  per IMPI basis includes information on whether each IMPI is a ‘primary’ or ‘secondary IMPI, and may advantageously include ‘barring’ indicators and respective implicit registration sets of contact addresses where each IMPI may be reachable. 
     In this respect, and for the sake of simplicity with regard to the exemplary IMS subscription  100  illustrated in  FIG. 2 , IMPU&#39;s  107  to  115  are supposed to be reachable in contact addresses IP.ad-1 to IP.ad-9, though other arrangements, including more than one contact address per each IMPU, are also possible. 
     Thus, as illustrated in  FIG. 3 , a first configuration element  121  is provided for the first IMPI  101  configured as ‘primary’ IMPI; associated with first IMPU  107 , third IMPU  109 , fourth IMPU  110 , fifth IMPU  111  and seventh IMPU  113 ; having a set of contact addresses comprising IP.ad-3, IP.ad-4, IP.ad-5 and IP.ad-7; and having no barring indicator active. A second configuration element  122  is provided for the second IMPI  102  configured as ‘primary’ IMPI; associated with second IMPU  108 , third IMPU  109 , fourth IMPU  110 , fifth IMPU  111  and seventh IMPU  113 ; having a set of contact addresses comprising IP.ad-3, IP.ad-4, IP.ad-5 and IP.ad-7; and having no barring indicator active. A third configuration element  123  is provided for the third IMPI  103  configured as ‘primary’ IMPI; associated with third IMPU  109  and fourth IMPU  110 ; having a set of contact addresses comprising IP.ad-3, IP.ad-4, IP.ad-5 and IP.ad-7; and having no barring indicator active. A fourth configuration element  124  is provided for the fourth IMPI  104  configured as ‘secondary’ IMPI; associated with third IMPU  109 , fourth IMPU  110 , fifth IMPU  111  and sixth IMPU  112 ; having a set of contact addresses comprising IP.ad-3, IP.ad-4, IP.ad-5 and IP.ad-6; and having a barring indicator active for own registration and for registration of any RSI. A fifth configuration element  125  is provided for the fifth IMPI  105  configured as ‘secondary’ IMPI; associated with third IMPU  109 , fourth IMPU  110 , seventh IMPU  113  and eighth IMPU  114 ; having a set of contact addresses comprising IP.ad-3, IP.ad-4, IP.ad-7 and IP.ad-8; and having a barring indicator active for own registration and for registration of any RSI. A sixth configuration element  126  is provided for the sixth IMPI  106  configured as ‘primary’ IMPI; associated with third IMPU  109 , fourth IMPU  110  and ninth IMPU  115 ; having a set of contact addresses comprising IP.ad-3, IP.ad-4 and IP.ad-9; and having a barring indicator active only for registration of any RSI. 
       FIG. 4  illustrates an exemplary configuration  130  at the HSS with configuration elements  131 - 139  per IMPU basis in accordance with the exemplary IMS subscription  100  illustrated in  FIG. 2 . This exemplary configuration  130  per IMPU basis includes information on whether each IMPU is associated with an RSI, or with an implicit registration set of IMPU&#39;s, and may advantageously include ‘barring’ indicators for call establishment. 
     Thus, as illustrated in  FIG. 4 , a first configuration element  131  is provided for the first IMPU  107 , not associated with any implicit registration set of IMPU&#39;s; not associated with any RSI; and not barred for call establishment. A second configuration element  132  is provided for the second IMPU  108 , not associated with any implicit registration set of IMPU&#39;s; not associated with any RSI; and not barred for call establishment. A third configuration element  133  is provided for the third IMPU  109 , associated with an implicit registration set of IMPU&#39;s that includes the third IMPU  109  and the fourth IMPU  110 ; associated with a first RSI  116  that includes the first IMPI  101 , the second IMPI  102 , the fourth IMPI  104  and the fifth IMPI  105 ; and not barred for call establishment. A fourth configuration element  134  is provided for the fourth IMPU  110 , associated with an implicit registration set of IMPU&#39;s that includes the third IMPU  109  and the fourth IMPU  110 ; associated with a second RSI  117  that includes the fourth IMPI  104  and the fifth IMPI  105 ; and not barred for call establishment. A fifth configuration element  135  is provided for the fifth IMPU  111 , not associated with any implicit registration set of IMPU&#39;s; associated with a third RSI  118  that just includes the fourth IMPI  104 ; and not barred for call establishment. A sixth configuration element  136  is provided for the sixth IMPU  112 , not associated with any implicit registration set of IMPU&#39;s; not associated with any RSI; and barred for call establishment, since it is a non-shared IMPU exclusively associated with a ‘secondary’ IMPI such as the fourth IMPI  104 . A seventh configuration element  137  is provided for the seventh IMPU  113 , not associated with any implicit registration set of IMPU&#39;s; associated with a fourth RSI  119  that just includes the fifth IMPI  105 ; and not barred for call establishment. An eighth configuration element  138  is provided for the eighth IMPU  114 , not associated with any implicit registration set of IMPU&#39;s; not associated with any RSI; and barred for call establishment, since it is a non-shared IMPU exclusively associated with a ‘secondary’ IMPI such as the fifth IMPI  105 . A ninth configuration element  139  is provided for the ninth IMPU  1115 , not associated with any implicit registration set of IMPU&#39;s; not associated with any RSI; and not barred for call establishment. 
     Other combinations and data distributions are also possible in the light of the above exemplary embodiments exemplified with configuration elements  120  and  130 . For example, where no implicit registration set of contact addresses is configured. 
     In operation, once a subscriber has registered in an access network and has gotten IP connectivity, such subscriber may register into the IMS network. To this end, the method illustrated in  FIG. 5  may be followed to register a subscriber with a UE  5  in the IMS network. In this respect, the present specification assumes the exemplary IMS subscription  100  illustrated in  FIG. 2  for exemplary describing the sequence of actions to be carried out in an illustrative and non-restrictive manner. 
     Conventionally, this method includes a step not illustrated in any drawing of configuring at a HSS  1  or  1   a , which holds subscriptions for subscribers of the IMS, the exemplary IMS subscription  100  for a subscriber with a number ‘n’ of IMPI&#39;s  101 - 106  and a number ‘m’ of IMPU&#39;s  107 - 115 , wherein each IMPI is associated with at least one IMPU and each IMPU is associated with at least one IMPI, and wherein some IMPU&#39;s  109 - 111  or  113  may be shared by more than one IMPI. 
     As illustrated in  FIG. 5 , this method starts with a step S- 100  carried out at the HSS  1  or  1   a  of configuring the IMS subscription for the subscriber with an implicit registration set  117  of ‘j’ IMPI&#39;s associated with a shared IMPU  110 ; and configuring an exemplary number of IMPI&#39;s  101 - 103  and  106  as ‘primary’ IMPI&#39;s, whereas the other IMPI&#39;s  104 - 105  are configured as ‘secondary’ IMPI&#39;s of this IMS subscription. 
     The method illustrated in  FIG. 5  continues with the explicit registration of the subscriber with UE  5  into the IMS. To this end, the UE  5  submits a register message during a step S- 105  towards a P-CSCF  4  for accessing the IMS network. This register message includes a given IMPI and a given IMPU to be registered during this registration process, and a contact address associated with the currently used UE  5 . In accordance with the invention, several embodiments are provided depending on whether the given IMPI is a ‘primary’ IMPI or a ‘secondary’ IMPI. 
     In a first embodiment, the IMS subscriber attempts to register with a given ‘secondary’ IMPI and a given IMPU. For example, where the UE  5  submits a register message during the step S- 105  towards a P-CSCF  4  including a given IMPI  104  and a given IMPU  110  to be registered during this registration process, and a contact address associated with the currently used UE  5 . 
     The P-CSCF  4  forwards such message during a step S- 110  towards an I-CSCF  3  of the IMS network where the IMS subscriber belongs to. The I-CSCF is in charge of selecting an appropriate S-CSCF for serving the IMS subscriber, and queries during a step S- 115  the HSS  1  or  1   a  with the given IMPI/IMPU pair and a network identifier of the P-CSCF  4 . 
     Assuming that the IMS subscriber had not previously registered with the given IMPI  104  and given IMPU  110  pair, the HSS  1  or  1   a  returns during a step S- 120  the capabilities required for an S-CSCF to be assigned for serving the IMS subscriber. The I-CSCF  3  receiving such capabilities selects an appropriate S-CSCF  2  fulfilling the capabilities, and forwards the register message during a step S- 125  with the given IMPI/IMPU pair and the contact address towards said S-CSCF  2 . The S-CSCF  2  receiving the register message submits during a step S- 130  its own registration towards the HSS  1  or  1   a  to indicate it has been assigned for serving the subscriber identified by the given IMPI  104  and given IMPU  110 . 
     The HSS  1  or  1   a , as receiving such indication of a registration of the subscriber with a given IMPU  110  and a given IMPI  104 , and an identifier of said S-CSCF  2 , determines during a step S- 135  whether the given IMPI  104  is configured as a ‘primary’ IMPI or as a ‘secondary’ IMPI; and, since the given IMPI  104  is a ‘secondary’ IMPI not previously registered by a ‘primary’ IMPI  101 - 103  within an implicit registration set, the HSS rejects during a step S- 140  such registration with the given IMPI  104  and given IMPU  110  towards the S-CSCF. 
     The S-CSCF receiving such rejection for the registration of the IMS subscriber with the given IMPI/IMPU pair, confirms back such rejection to the I-CSCF  3  during a step S- 145 , and this rejection is confirmed back from the I-CSCF towards the P-CSCF  4  during a step S- 150  and from the P-CSCF  4  towards the UE  5  during a step S- 155 . 
     In a second embodiment, the IMS subscriber attempts to register with a given ‘primary’ IMPI and a given IMPU. In this respect, the same sequence of actions illustrated in  FIG. 5  may be followed where the IMS subscriber attempts to register with a given ‘primary’ IMPI and a given IMPU. For example, where the UE  5  submits a register message during the step S- 105  towards a P-CSCF  4  including a given IMPI  103  and a given IMPU  110  of the exemplary IMS subscription  100  illustrated in  FIG. 2 , to be registered during this registration process, and a contact address associated with the currently used UE  5 . 
     The P-CSCF  4  forwards such message during a step S- 110  towards an I-CSCF  3  and the latter queries during a step S- 115  the HSS  1  or  1   a  with the given IMPI/IMPU pair and a network identifier of the P-CSCF  4 . Assuming that the IMS subscriber had not previously registered with the given IMPI  103  and given IMPU  110  pair, the HSS  1  or  1   a  returns during a step S- 120  the capabilities required for an S-CSCF to be assigned for serving the IMS subscriber. The I-CSCF  3  receiving such capabilities selects the S-CSCF  2  and forwards the register message during a step S- 125  with the given IMPI/IMPU pair and the contact address towards said S-CSCF  2 . The S-CSCF  2  receiving the register message submits during a step S- 130  its own registration towards the HSS  1  or  1   a  to indicate it has been assigned for serving the subscriber identified by the given IMPI  103  and given IMPU  110 . 
     The HSS  1  or  1   a , as receiving the indication of the registration of the subscriber with a given IMPU  110  and a given IMPI  103 , and an identifier of said S-CSCF  2 , determines during a step S- 135  whether the given IMPI  103  is configured as a ‘primary’ IMPI or as a ‘secondary’ IMPI; and, since the given IMPI  103  is a ‘primary’ IMPI, the HSS determines whether the given IMPU  110  is a shared IMPU associated with any implicit registration set of ‘j’ IMPI&#39;s. In the present case, the given IMPU  110  is the shared IMPU associated with the implicit registration set  117  of ‘j’ IMPI&#39;s, so that the HSS registers the ‘j’ IMPI&#39;s  104 - 105  along with the given IMPI  103  and, during a step S- 140 , downloads towards the S-CSCF the implicit registration set  117  of ‘j’ IMPI&#39;s. 
     In addition or complementary to these actions, the HSS may carry out at this stage the conventional steps of marking said IMPI&#39;s  103 - 105  and IMPU  110  as ‘registered’, storing a reference to the S-CSCF as been assigned for serving the IMS subscriber, and downloading a user profile associated with the given IMPU  110  towards the S-CSCF. Moreover, this user profile may also include a conventional Implicit Registration Set of IMPU&#39;s  109 - 110  associated with the given IMPU, as illustrated in the configuration element  133  of  FIG. 4 . Furthermore, the HSS may also download towards the S-CSCF  2  a set of contact addresses IP.ad-3, IP.ad-4, IP.ad-5 and IP.ad-7 associated with the given IMPI  103 , as illustrated in the configuration element  123  of  FIG. 3 . 
     The S-CSCF  2 , as receiving the implicit registration set  117  of ‘j’ IMPI&#39;s, may build up a data model  150 , exemplary illustrated in  FIG. 8 , and comprising: a set  151  of IMPI&#39;s that includes the given IMPI  103  explicitly registered and the implicit registration set of IMPI&#39;s  104 - 105 , a set  152  of IMPU&#39;s that includes the given IMPU  110  explicitly registered and an Implicit Registration Set of IMPU&#39;s  109 - 110 , and a set  153  of contact addresses, if any, for the given IMPI. 
     The S-CSCF receiving the user profile for the IMS subscriber and already having the given IMPI/IMPU pair and the contact address originally received in the registration message is now ready for serving the IMS subscriber. As illustrated in  FIG. 5 , this is confirmed from the S-CSCF  2  back to the I-CSCF  3  during a step S- 145 , and this confirmation is forwarded from the I-CSCF towards the P-CSCF  4  during a step S- 150  and from the latter towards the UE  5  during a step S- 155 . 
     In order to carry out the method illustrated in  FIG. 5 , there is provided an enhanced HSS  1  or  1   a , as illustrated in  FIG. 6  and  FIG. 7 , and an enhanced S-CSCF  2 , as illustrated in  FIG. 9 . 
     Thus, the HSS  1  or  1   a  illustrated in  FIG. 6  and  FIG. 7  comprises an accessible storage for configuring an IMS subscription for each IMS subscriber, wherein the IMS subscription includes more than one IMPI  101 - 106  and more than one IMPU  107 - 115 , wherein each IMPI is associated with at least one IMPU and each IMPU is associated with at least one IMPI, and wherein at least one IMPU  109 - 111  and  113  is shared by more than one IMPI. 
     This accessible storage is arranged for configuring the IMS subscription for the subscriber with an implicit registration set  117  of ‘j’ IMPI&#39;s associated with the shared IMPU  110 , wherein the ‘j’ IMPI&#39;s are preferably selected amongst the ‘n’ IMPI&#39;s in the IMS subscription, and for configuring at least one IMPI  101 - 103  or  106  in the IMS subscription as ‘primary’ IMPI and any other IMPI  104 - 105  as ‘secondary’ IMPI of the IMS subscription for the subscriber 
     In particular, as illustrated in  FIG. 7 , the accessible storage in the HSS may include an external database  10   a  acting as an HSS back-end shared by a plurality of HSS front-ends  1   a - 1   d , and a memory handler  11  for interfacing with the external database. Alternatively, as illustrated in  FIG. 6 , the accessible storage in the HSS may be provided as an internal memory  10 . 
     This HSS also comprises a receiver  50  for receiving from the S-CSCF  2  an indication of the registration of said subscriber with the given IMPU  110  and the given IMPI  103 , and an identifier of said S-CSCF; a processing unit  20  for determining whether the given IMPU  110  and the given IMPI  103  are associated, and wherein this processing unit is arranged for determining whether the given IMPI  103  is a ‘primary’ IMPI or a ‘secondary’ IMPI, whether a ‘secondary’ IMPI  104  has been previously registered within an implicit registration set, and whether the given IMPU  110  is the shared IMPU associated with the implicit registration set  117  of ‘j’ IMPI&#39;s. 
     Moreover, this HSS also comprises a sender  40  for downloading towards the S-CSCF  2  the implicit registration set  117  of ‘j’ IMPI&#39;s, where the given IMPI  103  is a ‘primary’ IMPI and the given IMPU  110  is the shared IMPU associated with the implicit registration set  117  of ‘j’ IMPI&#39;s; or for rejecting the registration of the subscriber with the given IMPU and given IMPI towards the S-CSCF  2 , where the given IMPI  104  is a ‘secondary’ IMPI not previously registered by a ‘primary’ IMPI within an implicit registration set. 
     Furthermore, the sender  40  of this HSS may be arranged for downloading towards the S-CSCF, along with the implicit registration set  117  of ‘j’ IMPI&#39;s, a set of contact addresses IP.ad-3, IP.ad-4, IP.ad-5 and IP.ad-7 associated with the given IMPI  103  and usable to reach at least one IMPI selected amongst the set  151  of IMPI&#39;s in a number of fixed and mobile devices. 
     Correspondingly, the S-CSCF  2  illustrated in  FIG. 9  comprises a sender  46  for submitting towards the HSS  1  or  1   a  an indication of a registration of an IMS subscriber with the given IMPU  110  and the given IMPI  103 , and an identifier of the S-CSCF; a receiver  56  for receiving from the HSS  1  or  1   a  the implicit registration set  117  of ‘j’ IMPI&#39;s associated with the given IMPU  110 ; a processing unit  25  for determining that the given IMPI  103  is a ‘primary’ IMPI of the IMS subscription; and an accessible storage  15  for storing a set of IMPI&#39;s  151  including the ‘primary’ IMPI  103  along with the IMPI&#39;s  104 - 15  in the implicit registration set  117  of ‘j’ IMPI&#39;s, and a set of IMPU&#39;s  152  including the given IMPU  110  explicitly registered. 
     Moreover, the receiver  56  of the S-CSCF may be arranged for receiving from the HSS  1  or  1   a , along with the implicit registration set  117  of ‘j’ IMPI&#39;s, a set of contact addresses IP.ad-3, IP.ad-4, IP.ad-5 and IP.ad-7 associated with the given IMPI  103 , and the accessible storage  15  may be arranged for storing the set of contact addresses in a set  153  of the data model shown in  FIG. 8 . 
     Generally speaking, the contact addresses are usable to reach at least one IMPI selected amongst the given IMPI and the set  151  of IMPI&#39;s in a number of fixed and mobile devices. To this end, S-CSCF illustrated in  FIG. 9 , may further comprise a second sender  47  arranged for using the contact addresses to reach at least one IMPI selected amongst the set  151  of IMPI&#39;s in a number of fixed and mobile devices  5 - 5   m . More precisely, the processing unit  25  of the S-CSCF may be arranged for instructing the second sender  47  to submit a message addressing the at least one IMPI towards a number of fixed and mobile devices  5 - 5   m  identified by the contact addresses received from the HSS  1  or  1   a.    
     In particular, the S-CSCF  2  illustrated in  FIG. 9  also includes a second receiver  57  for receiving during the step S- 125  the registration message originated from the UE  5  with the given IMPI  103 , the given IMPU  110  and the given contact address. 
     In particular, the S-CSCF  2  may be implemented so that the first and second receivers  56  and  57 , or the first and second senders  46  and  47 , are a same receiver  55  or sender  45  respectively, or even a unique input/output unit  35 . 
     Back to the first embodiment of the method illustrated in  FIG. 5 , whereby the registration of an IMS subscriber with a given ‘secondary’ IMPI  104  and a given IMPU  110  is rejected if the ‘secondary’ IMPI  104  had not previously been registered by a ‘primary’ IMPI  101 - 103 , this method may further comprise additional steps to avoid a terminating call reaching the ‘secondary’ IMPI. 
     As illustrated in  FIG. 10 , and upon receiving at the I-CSCF  3  an invitation to complete a terminating call, during a step S- 165 , for an IMS subscriber identified by a given second IMPU  114 , which is a non-shared IMPU associated with the ‘secondary’ IMPI  105  in the exemplary IMS subscription  100  shown in  FIG. 2 , the I-CSCF sends a query during a step S- 170  towards the HSS  1  or  1   a , inquiring about the subscriber identified by the second given IMPU  114 . As receiving this query, the HSS determines during a step S- 175  that the second given IMPU  114  is a non-shared IMPU associated with a ‘secondary’ IMPI  105  not previously registered by a ‘primary’ IMPI  101 - 103  within an implicit registration set; and rejects during a step S- 180  such query about the subscriber with the second given IMPU  114  towards the I-CSCF  3 . 
     In order to carry out the method steps illustrated in  FIG. 10 , an enhanced HSS  1  or  1   a  is provided wherein the receiver  50  may be arranged for receiving the query from the I-CSCF  3  about the subscriber identified by the second given IMPU  114 ; and, responsive to this query, the processing unit  20  may be arranged for determining that the second given IMPU  114  is a non-shared IMPU associated with a ‘secondary’ IMPI  105  not previously registered by a ‘primary’ IMPI within an implicit registration set; and the sender  40  may be arranged for rejecting the query about the subscriber with the second given IMPU  114  towards the I-CSCF  3 . 
     Back to the second embodiment of the method illustrated in  FIG. 5 , whereby the registration of an IMS subscriber with a given ‘primary’ IMPI  103  and a given IMPU  110  triggers the registration of an implicit registration set  117  of ‘j’ IMPI&#39;s  104  and  105 , this method may further comprise additional steps to terminate the registration of, at least, the ‘secondary’ IMPI&#39;s in the implicit registration set  117  of ‘j’ IMPI&#39;s. 
     As illustrated in  FIG. 11 , and assuming the IMPI&#39;s in the IMS subscription  100  illustrated in  FIG. 2  have been configured as ‘primary’ or ‘secondary’ IMPI&#39;s, the HSS  1  or  1   a  may receive during a step S- 190  an indication of deregistering a subscriber with a given IMPI  103  and a given IMPU  110  from the S-CSCF  2 . Then, the HSS may determine during a step S- 195  that the given IMPI is a ‘primary’ IMPI and the given IMPU is the shared IMPU associated with the implicit registration set  117  of ‘j’ IMPI&#39;s; and the HSS may deregister during a step S- 195  all those ‘secondary’ IMPI&#39;s  104 - 105  included in the implicit registration set  117  of ‘j’ IMPI&#39;s with any IMPU they had previously been registered. 
     Particularly applicable for other embodiments not illustrated in any drawing, namely where the implicit registration set of ‘j’ IMPI&#39;s includes ‘primary’ and ‘secondary’ IMPI&#39;s, the HSS may deregister during the step S- 195  all those IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s and not only those configured as ‘secondary’ IMPI&#39;s. 
     In order to carry out the method steps illustrated in  FIG. 11 , an enhanced HSS  1  or  1   a  is provided wherein the receiver  50  may be arranged for receiving from the S-CSCF  2  an indication of deregistering a subscriber with the given IMPI  103  and the given IMPU  110 ; and, responsive to this deregistration, the processing unit  20  may be arranged for determining that the given IMPI  103  is a ‘primary’ IMPI and the given IMPU  110  is the shared IMPU associated with the implicit registration set  117  of ‘j’ IMPI&#39;s; and wherein both the sender  40  in cooperation with the processing unit  20  may be arranged for deregistering towards the S-CSCF  2  all those ‘secondary’ IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s with any IMPU they had previously been registered, or all those IMPI&#39;s included in the implicit registration set of ‘j’ IMPI&#39;s and not only those configured as ‘secondary’ IMPI&#39;s, choice which may be determined by a first configurable parameter set during the configuration step. 
     In another embodiment of the invention not illustrated in any drawing, there is provided a second configurable parameter set during the configuration step to determine whether just the exemplary RSI  117  associated with the shared IMPU  110 , which includes the fourth IMPI  104  and the fifth IMPI  105 , is the only one to be implicitly registered, or also those RSI&#39;s associated with other shared IMPU&#39;s in a same Implicit Registration Set of IMPU&#39;s are going to be implicitly registered as well. For example, as shown in the configuration element  134  of  FIG. 4 , there is an Implicit Registration Set of IMPU&#39;s consisting of the third IMPU  109  and the fourth IMPU  110 ; then, depending on this second configurable parameter, just the RSI  117  associated with the given IMPU  110 , which includes the fourth IMPI  104  and the fifth IMPI  105 , is implicitly registered, or also the RSI  116  associated with the given IMPU  109 , which includes the first IMPI  101 , the second IMPI  102 , the fourth IMPI  104  and the fifth IMPI  105 , is implicitly registered as well. 
     On the one hand, as anticipated above, this method may further comprise a step of configuring at the HSS each ‘secondary’ IMPI of the IMS subscription as ‘barred’ for registration and, in particular, this step of configuring each ‘secondary’ IMPI as ‘barred’ for registration may include a step of barring for own registration the ‘secondary’ IMPI. This step is preferably made during the step S- 100  of configuring the IMS subscription for the subscriber, but it may be carried out at any time during operation. Where this barring is implemented, the determination carried out during the step S- 135  of this method, illustrated in  FIG. 5 , that the given IMPI  103  is a ‘primary’ IMPI and the given IMPU  110  is the shared IMPU associated with the implicit registration set  117  of ‘j’ IMPI&#39;s further comprises a step not illustrated in any drawing of unbarring for own registration those ‘secondary’ IMPI&#39;s  104 - 105  included in the implicit registration set  117  of ‘j’ IMPI&#39;s, that is a reset of the registration barring ‘OWN’ in the configuration elements  124  and  125 , preferably carried out by the processing unit  20  of the HSS actuating on the accessible storage  10  or  10   a . More precisely, where the accessible storage is arranged for barring an own registration of the ‘secondary’ IMPI, the processing unit may be arranged for barring and unbarring such registration barring in the configuration  120  of the accessible storage. 
     On the other hand, and in order to avoid that a kid previously registered by the parents further registers other kid prevented by the parents from registration, the step of configuring each ‘secondary’ IMPI as ‘barred’ for registration may also include a step not illustrated in any drawing of barring any ‘secondary’ IMPI for registration of the implicit registration set of ‘j’ IMPI&#39;s associated with the given IMPU. To this end, the configuration elements  124  and  125  shown in  FIG. 3  include a registration barring set to ‘RSI’. Moreover, not only a kid previously registered is prevented from registering other kids, but some other family member having a ‘primary’ IMPI assigned may be wanted to be prevented from registering any implicit registration set of ‘j’ IMPI&#39;s. To this end, this method may further comprise a step not illustrated in any drawing of configuring at the HSS a ‘primary’ IMPI  106  of the IMS subscription as ‘barred’ for registration of any implicit registration set of ‘j’ IMPI&#39;s associated with a shared IMPU. To this end, the configuration element  126  shown in  FIG. 3  includes a registration barring set to ‘RSI’. These steps of barring any ‘primary’ or ‘secondary’ IMPI for registration of the implicit registration set of ‘j’ IMPI&#39;s may preferably be made during the above step S- 100  of configuring the IMS subscription for the subscriber, but they may be carried out at any time during operation. In principle, this barring for registration of any implicit registration set of ‘j’ IMPI&#39;s, dislike the barring for own registration, should preferably not be reset after the implicit registration, but the processing unit  20  and accessible storage  10 - 10   a  of the HSS may also be arranged to this end. 
     Moreover, in order to support an effective control over call establishment procedures, the method may further comprise a step of configuring at the HSS each non-shared IMPU associated with each ‘secondary’ IMPI of the IMS subscription as ‘barred’ for call establishment. To this end, and in accordance with the exemplary IMS subscription illustrated in  FIG. 2 , the exemplary configuration  130  of  FIG. 4  shows the configuration elements  136  and  138  respectively provided for configuring the sixth IMPU  106  and the eighth IMPU  108 , which are non-shared IMPU&#39;s respectively associated with the ‘secondary’ IMPI  104  and with the ‘secondary’ IMPI  106 , wherein these configuration elements  136  and  138  indicate the sixth IMPU  106  and the eighth IMPU  108  as barred for call establishment. 
     This step of configuring each non-shared IMPU associated with each ‘secondary’ IMPI of the IMS subscription as ‘barred’ for call establishment may preferably be made during the above step S- 100  of configuring the IMS subscription for the subscriber, but it may be carried out at any time during operation. 
     Where this barring is implemented to control the call establishment procedures, the determination carried out during the step S- 175  of the method shown in  FIG. 10 , that the second given IMPU  114  is a non-shared IMPU associated with a ‘secondary’ IMPI  105  not previously registered by a ‘primary’ IMPI  101 - 103  within an implicit registration set, may be carried out by encountering that the IMPU  114  is ‘barred’ for call establishment in the configuration element  138 . Correspondingly, the determination carried out during the step S- 135  of the method illustrated in  FIG. 5 , that the given IMPI  103  is a ‘primary’ IMPI and the given IMPU  110  is the shared IMPU associated with the implicit registration set  117  of ‘j’ IMPI&#39;s, further comprises a step not illustrated in any drawing of unbarring for call establishment the non-shared IMPU&#39;s  112  and  114  associated with each ‘secondary’ IMPI  104 - 105  included in the implicit registration set  117  of ‘j’ IMPI&#39;s. That is, a reset of the barring for call establishment in the configuration elements  136  and  138 , preferably carried out by the processing unit  20  of the HSS actuating on the accessible storage  10  or  10   a . More precisely, where the accessible storage is arranged for barring the non-shared IMPU&#39;s associated with each ‘secondary’ IMPI, the processing unit may be arranged for barring and unbarring such barring in the configuration  130  of the accessible storage. 
     Furthermore, where the barring for call establishment of each non-shared IMPU associated with each ‘secondary’ IMPI, where the barring for own registration of each ‘secondary’ IMPI, or where both are implemented, the deregistration of ‘secondary’ IMPI&#39;s, which is carried out as a result of the determination made during the step S- 195  of the method illustrated in  FIG. 11 , includes a step of barring for call establishment in the configuration  130  each non-shared IMPU associated with each ‘secondary’ IMPI, and a step of barring for own registration each ‘secondary’ IMPI in the configuration  120 . 
     The invention may also be practised by a computer program, loadable into an internal memory of a computer with input and output units as well as with a processing unit. This computer program comprises to this end executable code adapted to carry out the above method steps when running in the computer. In particular, the executable code may be recorded in a carrier readable medium. 
     The invention is described above in connection with various embodiments that are intended to be illustrative and non-restrictive. It is expected that those of ordinary skill in this art may modify these embodiments. The scope of the invention is defined by the claims in conjunction with the description and drawings, and all modifications that fall within the scope of the claims are intended to be included therein.