The company I co-founded and manage since 2009 provides a solution similar to Slack, but white-label.
Taking a closer look at your requirements in your question comment:
- We deliver a dedicated standalone white label mobile app within 2 working days
- We provide an open REST API
- The app handles different roles to manage the entire data and the channels
- The app is available for Android phones and tablets, as well as iPhones and iPads
- The app has a web version too
- We’re around since 2009 and have clients all over the world.
Feel free to have a look at our website: https://minsh.com. There is also a free demo app available to test the various functionalities.
One more thing: my partner recently published an article which compares Minsh to Slack. Might be interesting to have a look and see which solution suits your client best.