In one embodiment, a scheme is disclosed for routing a Session Initiation Protocol (SIP) call in a network environment including a circuit-switched (CS) network and an IP multimedia subsystem (IMS) network. When a SIP call is originated by a user equipment (UE) device in the CS network domain, call information associated with the call is provided to an application server (AS) node disposed in the IMS network. At the AS node, a pool of E.164 numbers are maintained as IP multimedia routing numbers (IMRNs) which are mapped to or otherwise associated with called party SIP URIs. The AS node dynamically allocates a select IMRN with respect to a called party's SIP URI received from the UE device and returns it to the UE device. The dynamically allocated IMRN is then utilized for routing the SIP call towards the called party.