I'm porting or transferring a number (561-314-4575) from Google Voice to RingCentral. I already have an account with Ring Central with I think 2 numbers I haven't used at all (new account).

On the transfer step by step it asks for my billing phone number and then below I can type in the 20 numbers I wish to transfer.

What number do I type in the billing phone number box? One of the Ring central ones? Or the Google Voice one? I think I need to put the 561-314-4575 (the one I want to transfer) one in the box (that takes up to 20 numbers). I can't put it in both.