A method and device enable establishing an inter-radio frequency subsystem interface (ISSI) unit-to-unit call between a calling unit of a calling home radio frequency subsystem (RFSS), and a called unit of a called home RFSS, through a hosted ISSI function (HIF). The method includes processing, at the HIF, a first invite message received from the calling home RFSS, wherein the first invite message includes a first request uniform resource identifier (URI) that designates an alias system unit identification (SU-ID). In response to the first invite message, transmitting, from the HIF to the called home RFSS, a second invite message, including a second request URI that designates a real SU-ID, wherein the HIF acts as the calling home RFSS. The unit-to-unit call is thus established between the calling unit and the called unit through the calling home RFSS, the HIF, and the called home RFSS.