Once hired on a contract you are able to communicate with a client however you would like. But, if you are going off-system make sure it's something like Slack where you can easily track/save messages or screenshots from just in case.
The business to customer/user interactions are much more fluid on WhatsApp. It's extremely simple to use and one of the widest growing apps. That being said, I don't ever use it. The only one I will use besides slack or email is Telegram since it's big in the crypto space.
