ADF4156 start-up voltage

Hello

I'm a DFAE in Japan. The customer wants to start up ADF4156 at the lowest possible voltage due to his system issue.
According to the specifications, it is 2.7V to 3.3V, but what is the voltage of VDD when the IC starts up? Is it 2V, 1V or any voltage?
Since the customer guarantees the operation on the system, it seems that he wants to know the starting voltage as a guide.

Regards,

Hiroyuki