I face the same problem. The thing is, that on the Fanvils WebUI you can select and chose webhooks as an key option. This allowed me to send URLs to home assistant by pressing one of the blf keys. So the staff on the counter can easily unlock the door or move the window blinds. This cannot be done with the settings 3CX provides.
The approach must then be a bit different to create a custom template, I think?. But this is a bit user undfriendly and many of us would appreciate it if 3CX could add a feature to partially auto provision phones (or maybe include more option).