A method and apparatus for performing an inter-system soft handoff is described. In accordance with the present invention, when a subscriber unit crosses from a first cellular system to a second cellular system, a base station controller determines if sufficient network resources are available to conduct a inter-system soft handoff. If so, the base station controller generates a set of signaling messages that cause call processing resource to be allocated and for the call to be processed at the second cellular system. The base station controller then perform data-selection and data-broadcast for the call by transmitting data to the subscriber unit by way of the second cellular system as well as via one or more base stations to which the base station controller is directly coupled. The determination as to whether sufficient network resources are available to conduct the inter-system soft handoff is based on the type of connection that exists between the first cellular system and the second cellular system, the number of inter-system calls being conducted, and the frame offset of the call currently being processed.