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General Call Address is not intended to get the slave device ID's connected to the bus.
General Call Address is just used to address or communicate with all the device connected to the I2C bus at the same time. But, the catch is that the devices need to be intelligent enough to be able to process the bytes of data from the general call address.
The master is not aware of the number of devices connected to the bus. If a device wants the data or can process every byte of the call, it will act as a slave-receiver and if not, the device ignores the call entirely and doesn't send an acknowledgement.