The solution for this is simple. At the start of the competition say:
"You must be ready when your name is called. You will receive a DNF if you do not come immediately when called."
When the time comes:
"John Smith, you have 5 seconds to report to the table, or you will receive a DNF for your solve. 1, 2, 3, 4..."
This is exceedingly harsh. Especially at German Nationals (assuming I'm thinking of the right competition), the competition waiting area was very busy and pretty much standing-room-only, and it was also in the middle of a very busy festival. So if you were ready but perhaps at the other end of the crowd you might take longer than 5 seconds to get through to the table. Also, personally I also had to help out at the V-cube booth, so I'm sorry if I was late to some things, but it really was impossible for me to stand there all day.
I think a better solution would be to just ask every competitor who is in the middle of a round to sit in a waiting area behind the stage. This ensures that they will be in the area when called and makes it easy to see if someone is missing. Of course this requires a bit of extra space (although in any kind of auditorium this space would already be available), but if at all possible it is better to have a separate non-busy competitor's area near the tables.