How to capture ip address of watsapp number

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No way. Even if they wouldn't care about users' privacy, all these services have to use mediation servers for technical reasons - most clients are on private or "shared" subnets behind a NAT so nothing like "IP address of whatsapp number" actually exists, in most cases it is a set of at least two addresses both of which may dynamically change.

So if you're dealing with an abuse case, you have to talk about it with whatsapp's support.