XMPP. A business can self-host, there are public servers, or there are many businesses which offer customised xmpp hosting as a service.
I can be federated with other xmpp servers or be a locked-down work-only service, or federate with chosen other servers (such as a client company’s xmpp servers).
If that’s the main problem then that’s easy to solve! Simply use a free public xmpp server.
I mention the self- and paid-hosting options because businesses tend to like having a sevice agreement backed by a contract, and may have additional specialised requirements not provided by free services (xmpp or otherwise).