No you don't need to. Just install whatsapp and register with our phone number. When it asks for permission to contact list just decline if you don't want to sync your contacts to whatsapp. If you do though, then you can send whatsapp messages to any of your contacts that are also using whatsapp.

But not syncing does not prevent you from sending messages to anyone. Just put in the finnair number and send message.

I just installed it because it is the only electronic contact channel that works, even for platinum and I am tired of waiting in line. It has worked really really well. They answer almost immediately

It is sad that others than platinum can't get service at all.

As a comparison I just called Aegean customer service and it sure was nice to hear the words "The average waiting time is 1 minute" and it was true.