A mobile device associated with at least first and second subscriber identities and comprising: a modem capable of receiving messages related to either one of at least first and second subscriber identities, but not simultaneously; a reception/transmission block coupled to the modem and configured to receive and/or transmit signals: and a control unit associated with said modem, wherein the control unit is characterised by: performing an ongoing communication activity associated with the first subscriber identity, wherein the ongoing communication activity is a background activity or a foreground activity, and wherein when the ongoing communication activity associated with the first subscriber identity is being performed the control unit is configured to: - determine (S2-1,S2-2) whether gaps (40, 44, 50, 54) not allocated for the ongoing communication activity associated with the first subscriber identity currently being performed coincide with activity occasions (41-42, 51-53) associated with the second subscriber identity; and, - if at least one gap (44, 54) not allocated for the ongoing communication activity associated with the first subscriber identity currently being performed coincides with at least one activity occasion associated with the second subscriber identity, cause the modem to perform at least one activity associated with the second subscriber identity in the at least one gap. a modem capable of receiving messages related to either one of at least first and second subscriber identities, but not simultaneously; a reception/transmission block coupled to the modem and configured to receive and/or transmit signals: and a control unit associated with said modem, wherein the control unit is characterised by: performing an ongoing communication activity associated with the first subscriber identity, wherein the ongoing communication activity is a background activity or a foreground activity, and wherein when the ongoing communication activity associated with the first subscriber identity is being performed the control unit is configured to: - determine (S2-1,S2-2) whether gaps (40, 44, 50, 54) not allocated for the ongoing communication activity associated with the first subscriber identity currently being performed coincide with activity occasions (41-42, 51-53) associated with the second subscriber identity; and, - if at least one gap (44, 54) not allocated for the ongoing communication activity associated with the first subscriber identity currently being performed coincides with at least one activity occasion associated with the second subscriber identity, cause the modem to perform at least one activity associated with the second subscriber identity in the at least one gap. - determine (S2-1,S2-2) whether gaps (40, 44, 50, 54) not allocated for the ongoing communication activity associated with the first subscriber identity currently being performed coincide with activity occasions (41-42, 51-53) associated with the second subscriber identity; and, - if at least one gap (44, 54) not allocated for the ongoing communication activity associated with the first subscriber identity currently being performed coincides with at least one activity occasion associated with the second subscriber identity, cause the modem to perform at least one activity associated with the second subscriber identity in the at least one gap.