Fast call set-up for a call to a multi-mode communication unit (102) is facilitated by a method and apparatus for registering and re-registering priorities with a SIP registrar (111). A list of contacts with first priorities associated with a mode of operation in a first enterprise network (106) is established and used. A switch to second priorities is made when a probability of a switch from the first mode of operation to a second mode of operation in a second cellular network (108) satisfies a threshold. The list of contacts includes a contact having an expiration time for the first mode of operation. A quality factor is determined and the list of contacts is re-registered and the expiration time changed if the quality factor does not satisfy a threshold.