Talkback only functions on moments when there is no sound received from the Baby Unit.
This is a one-way communication device only.
You can not talk to your baby at the exact same moment while receiving incoming baby sounds; that is technically not possible on this model.
Note: The Talkback function does work in full duplex mode on all new DECT baby monitor models. This is because the new DECT technology does allow two-way communication.