Actually, now I have just read back the replies, I realise that there may be a couple of wires getting crossed here.
Forgive me if I'm wrong, Blake, but you appear to be implying that, on occasion, having sourced a casting director, you have rung them up asking about the possibility of being seen for a specific project, and they have ultimately agreed to this.
This seems a logical achievement to me, because as I was just saying, if the casting director needs certain types, talents, skills etc., AND YOU OFFER THEM AT THE APPROPRIATE MOMENT, there is no reason that they will rule you out. Quite how they follow up on your suitability for the project in question I leave for Blake to explain: a casting director worth his/her salt will wish to make sure you don't call their reputation for good judgement into question at an audition, and so will be eager to assess you to some degree. At the very least, they should want to read your CV, perhaps watch a showreel, see you perform either in a separate project or at a personal meeting, face-to-face.
Perhaps, Blake, did you mean that the phone calls secured you the intial audition with the casting director?
Anyway, that's one thing. And, broadly, I agree with Blake; such approaches to CD's may be slightly unconventional, but how will they ever know you are out there unless you bring yourself to their attention? The idea that Edward has been talking about, however, seems to be more along the lines of asking the Casting Director, apropos of nothing much, whether they would be prepared to come and see another performance, assess a long-term suitability for being 'kept on file' etc. I think that this sort of thing can be done as effectively on paper as over the phone (indeed casting directors would prefer to have your CV in hand in such a scenario). Also, that nowadays, most CD's do not put themselves out any more than many agents do to attend performances - there is simply not enough time to do it, and those who do make it a point of principle to keep up with what is happening in theatre will generally go to the most high profile shows currently being developed.
The best chance of getting casting directors on side, I reiterate, is applying to them when they are specifically looking for certain types of actor to cast. Then, there is the chance that they will see you, remember you and take your application seriously.