If the interphone is U/S then cycling the seatbelt sign is an indication of an ALERT CALL (this could explain the expression on their faces). Meaning the CC will stop the service and go to their station and await the call from the SCCM. Meanwhile the SCCM and the nearest Crew member will go onto the flight deck and expect/take a NITS briefing (emergency brief).
If nothing followed then It could be as simple as a slip on the switch. Or my guess the pilots needed their dinners.
B