If we have an existing subscribed contact list for SMS, can we use WhatsApp to message these recipients?

Even if you already have a subscribed base for SMS, all outgoing WhatsApp messages must still start with an initial “Opt-In” by the user. This can be collected through any third party channel. For example in an SMS message, with a Web Form, in an Email, or even via a deep-link in print media. 

CommzGate portal provides convenient tools for you to register a contact as opted-in, including a feature to broadcast an WhatsApp opt-in hyperlink request to your existing subscriber base via SMS.