Well literally at the last minute our wonderful telephone engineer got the intercom system up and running.
In the end I went with "Teachers, time for your next appointment please - thank you". Simple, professional, effective apparently....but not as much fun as some of your suggestions! I made a point of using my 'bestest' Brit' accent, the posh clearly enunciated one for maximum effect and impact ;-)
It was interesting though, in the past a bell has been rung every 15 minutes, but we think an actual voice worked better at keeping them moving, as nobody got behind. We also had our best turnout ever, due in part we are sure to using the SchoolReach calling system to remind parents about it.
Anyway, a success, a very long 12-hour day, so I am drinking a hot cup of tea and going to bed with my book, the hubster and 3 cats!