S4B works with our VoIP infrastructure and is on-premise but the client is awfully slowly and badly needs to be reworked. But with Microsoft is abandoning on-premise services...

It's so slow and buggy that sometimes it registers clicks I do on the chat window as if I did them on the contact search one and end up sending that funny meme to a multichat I created with 30 I don't know instead of the contact/tab I selected.

SfB is ok for one-to-one or one-to-few communication, but the chats are pure garbage.

I consider Skype to be a dead product at this point.