What must be pressed on the interphone system when making regular calls?

Pressing the INTPH button on the interphone system is essential when making regular calls within the aircraft. This button activates the interphone system, allowing for communication between crew members in different positions, such as the flight deck and the cabin.

When the INTPH button is engaged, it connects to the standard communication channels used for routine discussions or announcements. This ensures that all parties can hear and respond appropriately during regular operations. It is specifically designed for this purpose, making it the correct choice for establishing non-emergency communication within the aircraft's systems.

Other options, while important in their respective contexts, do not facilitate standard communications like INTPH does. For instance, CAPT might be utilized for direct communication with the captain, EMER is typically reserved for emergency communications, and ALT might refer to altitude-related functions. However, none of these would allow for the same broad, regular communication as the INTPH button.