What must happen to the Interphone test according to protocol?

The requirement for the Interphone test to include all crew members is rooted in the necessity for effective communication and coordination among the cabin crew during flight operations. When all crew members participate in the test, it ensures that everyone is familiar with the intercom system and understands how to use it properly in case of an emergency or when relaying important information. This collective testing helps to identify any potential issues with the interphone system while also enhancing teamwork and preparedness among the crew. The involvement of every crew member ensures a shared level of understanding and readiness, which is vital for maintaining safety and efficiency during flights.