A lot of people have been asking about this. Here is some more data before you upvote.
WhatsApp has many more restrictions than Facebook Messenger.
You will have to pay extra for each account since it requires a dedicated server. (think $50-100/mo)
You will have to go through manual approval for the account. Conditions to get in are much stricter, so might not even qualify.
You can't send any promotional messages.
You have to pay for non-promo messages that are sent beyond 24 hours.
Moreover, you will have to go through manual approval for messages sent beyon 24 hours.
Basically, you need to provide a list of templates of messages that you plan to send beyond 24 hours and Facebook has to approve them, before you can send them.
This is NOT like Sponsored Messages.
You are not expected to be adding new templates all the time.
Think of this as transactional messages that first require an approval from FB.
Every message that you want to send beyond 24 hours has to be one of the templates that you got approved.
The good thing is - 24 hour window works the same way. You can do automation and live chat without limits.
That said, you don’t have buttons and quick replies on WhatsApp.
Only text and pictures.
Given all this - are do you still want WhatsApp? What is the use case for your/client's business?