Here's another way that CC get to solve their problem (at our cost, in time, in reduced wages and poorer conditions):
FO in Europe accidentally answers the phone in the morning on (say) Monday and agrees/gets talked into/gets intimidated into operating on Tuesday. He likes this because his published roster had him operating on the Monday, which is why he didn't worry too much about picking up the phone. "Cool", he thinks: extra day at home.
The rest of the original crew (now down to 3-men) turn up to work on Monday afternoon to find a message from CC that says: "So sorry Captain, your FO has called in sick, roster disruption, blah, blah, must operate 3-man to HKG".
He duly does this and this is how CC wins another round! (The only other available FO for Tueday's flight was into severe overtime).
DO NOT ANSWER THE PHONE! BREAK THE HABIT!