This is a very simple addon which lets you sync your Whmcs customers (phone and mobile number) to your gmail contacts.
This also will sync to your Whatsapp , as Google pushs your contacts to your phone as soon they are updated in you gmail account (should be the account that you have configured in your android phone)
This works only from whmcs to Gmail-Whatsapp
The lite version: will sync only your first 20 customers.
The paid version: will sync only your first 3.000 customers
The next version will be updated in real time using Google APIS , will be launched very soon.
Instructions, when you download it.