A method performed by a first node (111) managing a P-CSCF in a first communications network (101). The method is for handling identification of a second communications network (201). One of the communications networks is a home network and the other is a visited network. The first node (111) obtains (701) a unique identifier uniquely identifying the second communications network (201) in a plurality of networks (200) sharing a Mobile Network identifier. The unique identifier is based on: a) an MCC and an MNC, corresponding to a shared PLMN identifier, and b) a subscriber identifier corresponding to the second network (201). The first node (111) also processes (702) provision of an IMS service across the communications networks, to a device (230) operating in the visited network, based on the identifier. The processing (702) comprises, in the home network, one of: i) application of a policy or rule, and ii) routing a communication to a S-CSCF.Publ.