how to start a conversation

Okay girls , let me start off by saying  that i have a boyfriend who I've been dating for 4 months now and it's like In person we can talk about literally everything with no awkward silence but on FaceTime or on the phone I literally go blank. I forget how to talk and don't know how to start a convo I can literally be on FaceTime with him for 2 hours and stay quiet . Idk what to talk about if I'm a reserved person , I barely go out .... ?