A transition region can be defined for a first network segment that can provide wireless communication services using a first radio access technology (RAT) and a second RAT and that supports Internet protocol (IP) packet-based voice calls. The transition region can be defined based at least in part on a first coverage area of the first network segment and a second coverage area of a second network segment that does not support IP packet-based voice calls and does not support a first handoff mechanism based on the first RAT. A network node can be notified about an IP packet-based voice call involving a user equipment (UE) that is receiving service from the first network segment using a first RAT. Upon determining that the UE is located within the transition region, the network node can cause the UE to be switched to a second RAT for the IP packet-based voice call.