WhatsApp uses the same Internet data plan as web browsing and email on your phone. You might be charged for using WhatsApp and other data while you're roaming. Contact your wireless carrier for details about international roaming countries, SIM cards, rates and policies.
If you don't have an international data plan, you can avoid data roaming charges by turning off cellular data or data roaming:
Using WhatsApp while your phone is connected to Wi-Fi shouldn't incur charges from your carrier. Please note that some Wi-Fi networks might have charges or usage fees.
Even with your SIM card removed or your cellular connection disabled, you can continue using your existing phone number in WhatsApp when on Wi-Fi. If you have a local SIM card while traveling, you can use the change number feature to migrate your account to your new phone number.