It's not the best integration though. Usually when someone sends a message I get a notification to 'Mark as read' or 'Reply'.
The first reply I send is seemless. But it doesn't start a messaging window or anything like that. If that same person sends a follow up message shortly after then the notification doesn't really work the same way again, or at all. I forget.
Perhaps I'm meant to be running the SMS applicstion all the time, which is a seperate widget or something and part of kdeconnect. I usually don't run this and I think it was because I ran into other issues with it (I think it tries to sync all your phone messages so perhaps if you have a GB or two of messages it struggles).
What I might do is keep it running today while working and see if that makes the integration work better, allow me to send messages from the computer on the fly etc.