you can add additional transmitters/receivers, and you just need to
pair new transmitters or receivers to original doorbells one by one for a
new code. Following below steps:
1. Pick the sound you want on one of receivers.
2. Press and hold the volume button on this receiver about 5 seconds, the receiver will blink with a "ding" sound and then immediately push one of the transmitters to pair with the receiver. Finally you will hear the music you set.
The operation method is the same to pair other transmitters to receivers.