A method executed by a UE communicatively connected to a first 3rd Generation Partnership Project (3GPP) network utilizing an advanced Radio Access Technology (RAT) is provided. The methods includes the following steps: receiving a message from the first 3GPP network, which indicates that the first 3GPP network supports IP Multimedia Subsystem (IMS) voice over Packet Switched (PS) sessions; and providing an indication that fallback of the UE to a second 3GPP network utilizing a legacy RAT is disabled to the first 3GPP network in response to the receiving of the message.