You can pair transmitters with receivers one by one for a new code to expand transmitter buttons/receivers or get a ring tone.- Please pair transmitters with receivers one by one for a new code before you install the transmitters on the door frame. - If you have installed your transmitters on the door frame, please use the small screwdriver (included) to open the transmitter and take the front cover off, and take it close to the receiver to pair them.(Note: They can maintain their settings when power is removed, no need to reset).Here's how to pair them 1. Plug one receiver into a regular socket, and pick the sound you want on this receiver. 2. Press and hold the volume button on this receiver for 5 seconds or so, then the receiver will blink with a "ding" sound. And then push the transmitter button at the same time (within 5seconds) to pair with the receiver. Finally, you will hear the music you set. 3. The same way to pair other transmitter buttons with receivers.