Using an old phone number with Whatsapp on a phone with no SIM card
Here's what seems to have worked. The trick is that the system sends the same code via SMS and voice message. So in order, here's what happens. 


The SMS is sent to the old number and received.
The Android phone tries and fails for five minutes to autodetect the SMS.
Whatsapp on the new phone then offers to voice call the old number, and asks you to enter the number you hear.
The voice call will never be received, but you can enter the number sent in the SMS anyway.