When you are traveling out of the country, you can still use WhatsApp account via mobile data or Wi-Fi.

If you use a local SIM card while you are traveling, you can still use WhatsApp with your home number. However in this case, if you need to re-verify your account, you won't be able to do this while using the local SIM card. To re-verify / verify a phone number with WhatsApp, you must have the corresponding SIM card in your phone, with phone or SMS service enabled.