Methods and apparatus for originating a Session Initiation Protocol (SIP) call from a user equipment (UE) device in a network environment including a circuit-switched (CS) network and IP multimedia subsystem (IMS) network to a called party are disclosed. In one illustrative example, when the SIP call is originated by the UE device in the CS network domain, a SIP Invite message which includes a SIP Uniform Resource Indicator (URI) or Tel URI of the called party is sent from the UE device to the IMS network (e.g. to an application server (AS) node). At the AS node, a pool of E.164 numbers are maintained as IP multimedia routing numbers (IMRNs) which are utilized for mapping to or otherwise associating with called party URIs. Thus, the AS node dynamically allocates a select E.164 number with respect to the called party's URI received from the UE device, and returns it to the UE device in a SIP 380 (Alternative Service) Response message. Subsequently, the dynamically-allocated E.164 number is sent from the UE device in a call setup message for identification of the URI at the AS node, via the mapping, so that the SIP call may be properly routed towards the called party.