Systems and methods for a Session Initiation Protocol (SIP) router are described. In one aspect, the SIP router receives a SIP INVITE for an incoming call to a target mobile subscriber. Responsive to receiving the SIP INVITE, the SIP router identifies, via a Send Routing Information for Short Message (SRI/SM) request to a Home Location Register, a first address of a serving switch in a circuit-switched mobile network; a mobile telephony device associated with the target mobile subscriber is currently location-updated on the serving switch. The SIP router maps the first address to a second address for the serving switch; the second address being a SIP address. Using the SIP address, the SIP router sends the received SIP INVITE to a SIP interface of the serving switch. The serving switch being configured to process the received SIP INVITE and the incoming call to the target mobile subscriber.