Can a number that hasn't been registered on WhatsApp appear on other people's WhatsApp contact list as registered and available? A friend of mine uses two numbers, one of them is active on WhatsApp and as such the other wasn't. I then found out a couple of days ago that his other number appeared on my WhatsApp chatable list which got me surprised. After I asked him about it, he said he never had the number registered. I know for one that if anyone, under normal circumstances, was to have a number registered there, an OTP would be sent to the number which is to be used on it to verify or complete the process.
Can a number that hasn't been registered on WhatsApp appear on other people's WhatsApp contact list as registered and available?
No. If the number was never used in WhatsApp, it will not appear as registered.
It's possible that the person who had your friends number in the past, (phone numbers get recycled) made a WhatsApp account, and never deleted it, therefore it appears as if that number is registered to WhatsApp.