I think you need to remind her every time she drops by that it is important that she call first.
.... AND that you answered with an afirmative that she could come by. You don't want to set a precedent of "well, I CALLED like you asked! It's not my fault you didn't answer the phone!"
There are some ILs that try to fudge the line. Don't let her steamroll you in to answering every knock. If you don't want to answer, you do not have to invite them in. It's ok to answer the phone and say "now is not a good time." If you're feeling generous you or your DH can suggest an alternate, but again, you don't have to be the gatekeeper. This IS DH's mom, afterall. He could step up and handle his parents as he sees fit.