how can i make sure the AI does not speak when its called. it waits for user to speak ? is it possible to do in Vapi ? I see that Retell allows it https://cdn.discordapp.com/attachments/1211484481848873020/1307774386870161591/image.png?ex=673b874f&is=673a35cf&hm=f57759509eaacc82175baf2e9cb92336709e636e1ad741f973b91d68df6e2676&