This is similar to our overall Messaging System per https://dev.blesta.com/browse/CORE-2806

I think what Eddris Khan means is that an SMS message will be sent to any client's that have What's App .

Admittedly, I have some client's that I have to text regularly concerning paying their bills on time because they are too lazy to check their emails more than once a year.

So contacting them by SMS is also a good mode of communicating / contacting with them too concerning their bills and service notifications .

i don't ware of any API that allow you send messages to whatsapp number directly, you should go with a third party services.