Find proper 3rd party applications to integrate WhatsApp into current chat system, $20

We have a chat system based on MySQL and PHP. For each logged in users, a chat button appears on their company profile page. Guests can click the button to start to chat with them by given their name. Both logged in users and guests have unique ID mark under the hood. Now we want to integrate WhatsApp into our chat system. When a registered user linked their WhatsApp account, their chat button always shows on their company profile page. So a guest can chat to them anytime. Messages sent by guests will be forwarded to their WhatsApp account when they are not logged in yet. Messages they reply on the WhatsApp will be forwarded to the guest. We want registered users to be able to chat with multiple guests by WhatsApp at the same time, but if it's hard, can limit it to 1 guest at a time. This task is to review what 3rd party applications we should use - mention a few good options and recommend the one you think is best. Also mentioning any other tips you think we should be aware of when we are working to integrate this feature. Fixed price job - $ 20. Please start your application with "Verona" so we can see you read the job description and understand the terms.