A UE determines a set of RATs available in accordance with a first subscriber identity associated with a first network and a second subscriber identity associated with a second network. When the set includes a first RAT accessing a base station in the first network, a second RAT accessing a base station in the second network, and a third RAT accessing an access point of a WLAN, the UE selects, in a first performance configuration, the first RAT to establish a first call under the first subscriber identity. The UE further communicates, under the first subscriber identity, with a first IMS server of the first network through the first RAT to establish the first call. The UE maintains, under the second subscriber identity, a connection with a second IMS server of the second network through the first RAT or the third RAT when the first call is active.