yes! We trained Shaye to do this, because we could not see when she was standing at one of our doors. We hung two bells on lengths of ribbon and put one around the door handle of the back door, and one around the handle of the sliding door. Each time Shaye indicated she wanted to go out, or when we decided she should go out, we took her paw and hit the bell with it. When she ran to the door to go out, we'd ignore it once or twice and then go with her and help her ring the bell - in less than a week she had caught on and began ringing the bell herself so we'd know she was at the door. If your dog keeps ringing the bell only when it really needs to go, it's a wonderful assist, especially if you can't see all the doors, like we can't.
HOWEVER, one word of caution: After a while, Shaye began ringing the bell all the time so she would be taken out, whether she needed to go or not, just to run around. We fell for it at first, then realized she was playing us, and now both girls go out on a regular schedule; we have figured out how long they will hold, and what they need to do when. So, no more bells.