Before call handover between radio network subsystems (RNS's) of a radio access network, a serving radio network controller (RNC) of a first RNS determines the level of congestion in an Iur interface between the serving RNC and an RNC of a second RNS. If the Iur interface is determined to be congested when a call handover is required, the serving RNC uses the Iu interface with a core network of the radio access network to signal the core network to perform a call handover, and the core network uses the Iu interface with the RNC of the second RNS to signal the RNC of the second RNS to become a serving RNC for the call.