There are a lot of good resources out there. Here are a few I found,
From the first link: An Chat API providers & Second link : Video & Voice call providers
You can choose the best developers from above these posts for your business.
Using the pusher module form the appstore will help you setup a chat function in minutes, see https://appstore.home.mendix.com/link/app/107957/Mendix/Pusher
You can take a look at the Chatter module from the AppStore. It only facilitates one on one chats, but it's the closest you'll get. Be aware that the module does not have platform support: Mendix won't help you solve any problems, but the community might be able to help you.
Well, that’s a tremendous idea to create an app like WhatsApp, But you have to get to know the technologies and features used to build it.check this article it will realy help you more understanding about technology https://morioh.com/p/1b93f669f47e