For an incoming session intended for a user element, a domain selection function (DSF) is provided in a multimedia subsystem (MS) to select a circuit-switched subsystem (CS) or MS domain to use when routing the incoming session to the user element. Regardless of the domain in which incoming sessions are originated, the incoming sessions intended for the user element are routed to an S-CSCF in the MS. The S-CSCF directly or indirectly employs the DSF to determine whether to use the MS or the CS for terminating the incoming session. Based on available domain selection criteria, the DSF will select an appropriate domain, such as the MS or CS, to use for routing the incoming session to the user element. The domain selection decision of the DSF is provided to the S-SCSF, which will proceed by routing the incoming session to the user element via the selected domain.