I've worked for a few professional theatres, and all have been using the space up until the half-hour call. Whether it's the cast warming up, doing fight call, or running scenes or moments, or the crew working on any number of technical aspects, there has something been going on.
People don't sit quietly backstage waiting for the house to open.
"...everyone finally shut up, and the audience could enjoy the beginning of the Anatevka Pogram in peace."