It is usually very difficult to train dogs to go outside for their potty needs. You can use a belly belt which has a small pad or maxi pad in the centre and it comes around with velcro fastens on the back. You can train the dog with this belt. You need to be very consistent as it might take 6-8 weeks for this training.

 I am curious to know how you get the dog to actually bark at the door. Is this a natural trait that some dogs do or is there training involved?

 I have been going through the same thing with my dog. He just stands quietly by the door. Most of the time I notice because he is not in my site playing or sleeping. I thought him how to speak on command some time ago. So every time I brought him out and back in I would have him speak before I opened the door. It didn't take though, that is my fault. I think if I get consistent it may just work.