A method performed within a communication system comprising a 5G core network and an IP Multimedia Subsystem (IMS) network, where the 5G core network is configured to implement a plurality of virtual core network slices each having a slice identifier. The method comprises assigning one of said virtual core network slices to a User Equipment (UE) at or following registration of said UE with the IMS network, and sending from the UE or the 5G core network, to the IMS network, the slice identifier of the network slice assigned to the UE. The method further comprises receiving the slice identifier within the IMS network and storing in the IMS network a mapping between the UE and the received network slice identifier. The slice identifier is used within the IMS network to provision services to the UE.