Similar to Juan's reply - our (JAA operator) standard call in an urgent situation such as this is "Purser report to cockpit" and has to be done on the PA in case cabin crew are busy in the cabin pressing the normal 2 chime bells means they first finish what they are doing (serving meals/selling sandwiches) and only then come up & call you back - wasting precious time in an urgent situation.
What urgency call does your company have brit bus driver?