You don't need whatsapp to do wifi calling back and forth to US. Put your phone on airplane mode before landing. Connect to wifi (which I assume you will have where you are staying), turn on wifi calling (Google it if you are not sure how to do) and you are good to call and text numbers back in the states. If you are not in airplane mode before landing, your phone will automatically connect to the closest available cell tower and you will be charged for any text or call downloaded to your phone.