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Only devices without a keypad (such as Bluetooth headsets) have a default Bluetooth passcode (or pairing code).
The phone has no defaults, but when pairing two Bluetooth devices, you enter the same code on both devices (and if the other one has a fixed passcode, then on your phone you enter whatever the fixed code is).
When it is a fixed code, it can be 0000, 1234 or whatever the device manufacturer decided (it can also vary for each individual device manufactured, too, in which case the code is usually printed on a separate sticker or paper).
The passcode is only used once to establish the Bluetooth encryption/decryption keys for the two devices to communicate. The code is not used thereafter by the devices when they talk to each other (and is not saved/stored, either).