For on-prem, why not use a free and enterprise-quality XMPP server, like Openfire[1]?
It's Java so runs just about anywhere, and with even relatively low resource allocations, you can support a large number of concurrent chats and group sessions. Plus, since XMPP is an Open Standard, you can use any XMPP complaint client of your choosing.
Cisco embeds Openfire in a few of their enterprise products already (including Finesse), so you might have used it without even knowing.
The IgniteRealtime community is pretty active and new releases come out regularly[2].
It's Java so runs just about anywhere, and with even relatively low resource allocations, you can support a large number of concurrent chats and group sessions. Plus, since XMPP is an Open Standard, you can use any XMPP complaint client of your choosing.
Cisco embeds Openfire in a few of their enterprise products already (including Finesse), so you might have used it without even knowing.
The IgniteRealtime community is pretty active and new releases come out regularly[2].
[1] https://www.igniterealtime.org/projects/openfire/
[2] https://www.igniterealtime.org/index.jsp