Sony MDR-ZX780DC Help Manual page 38

When the headset is initialized, all pairing information is deleted.
In this case, delete the pairing information of the headset from the BLUETOOTH device, and then pair them again.
If the pairing information was deleted by initializing the headset, etc., delete from your iPhone the pairing information for the headset, and then pair
them again.
The headset can be paired with multiple devices, but can only play music from one paired device at a time.
Hint
The above procedure is an example. For details, refer to the operating instructions supplied with your iPhone.
To delete all pairing information, see
Related Topic
How to make a wireless connection to BLUETOOTH devices
Connecting to a paired iPhone
Listening to music from a device connected via BLUETOOTH
Disconnecting BLUETOOTH connection (after use)
Initializing the headset to restore factory settings
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"Initializing the headset to restore factory settings."
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