This was not our experience, or I may be misunderstanding what you mean. Our TA received an email confirmation after we got back home from our cruise from DCL with the placeholder info. In the meanwhile I had chosen from dcl's website which cruise and which stateroom we wanted. I also emailed that information to our TA. And that same day, probably in less than an hour, we had our entire reservation taken care of. No phone calls necessary. We had however used our same travel agent for the cruise we were currently on, so that info is saved in the DCL system. If you are not using a TA on the current cruise, it is a good idea to know which TA you will be naming to the placeholder before you board.
One reminder however, I believe you do need to transfer a placeholder to a travel agent within 30 days. I'm not sure about that since we did it a few days after our cruise, so someone may have updated information regarding that.