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Your idea regarding the area code is relevant. It is indeed possible that WhatsApp haven't included the new area code, even if I think it normally should... If you've another phone number with that same area code, you can test it with WhatsApp to see if you obtain the same error.
I also see another possibility. Let's suppose that phone number was owned by someone else in the past, and used WhatsApp in a manner that goes against the rules of the platform. They blocked that number and it is no longer possible to use it on WhatsApp. That person unsubscribed from their mobile service provider then. Today, that same number that's blocked from using WhatsApp has been assigned to you when subscribing, causing this registration error.