Using an old phone number with Whatsapp on a phone with no SIM card I have the following situation, which though similar to other questions I believe not to be a duplicate. 


I currently own a rather old iPhone with a SIM card installed and a new Android phone without. (There is a Verizon SIM card for the new phone, but I am leaving it unactivated it to avoid penalties from Verizon, which which I have no contract.)
I am in the US now on vacation, but new Android phone is intended to accompany me back where I live and not see service in the US in the foreseeable future. 
I can't install my old SIM card on my new phone because it is the wrong size, and I am afraid of destroying it if I try to cut it down to nano-SIM size myself. (I guess I'll have to transfer my number to a new nano-SIM card from my provider when I arrive in my home country again.)


Now I would like to be able to use Whatsapp via Wi-Fi on the new phone with my old number and hence appear as my familiar old number to my old contacts. But Whatsapp demands, in order for me to use that number, that I have that SIM card on my new phone. If you send a SMS, rather than letting you respond from the old phone, it tells you it is waiting to autodetect the text on the new phone. When that fails, it offers to voice call the old number, but that number has no service in the US.

I am capable, on the other hand, of registering Whatsapp as a landline, through the voice call option, but this doesn't let me reach my old contacts through Whatsapp, and trying to change to my own number on this app activates the same sequence of countermeasures enumerated in the previous paragraph.

There are numerous solutions posted online, but most are of out of date and seem to be defeated by the current version of Whatsapp, specifically the facts that the SMS must be auto-detected or the voice call must be picked up. The most up-to-date solutions I've found suggest one log into Whatsapp on the web through the browser on the old phone to obtain a QR code scannable on the new phone, but the Whatsapp-on-the-web feature is unavailable to my old iPhone's OS, and upgrading to iOS 8.1 or higher would brick that phone.

But I'm still not willing to concede defeat. What can I do?
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.