1)I am returning a call with regard to / regarding an inquiry I made from this number about the Family Day opening hours.
2)I am calling with regard to / regarding the inquiry made from this number a few days ago about the opening hours on Family Day.
3)We have two rooms with different keys. The number on each key corresponds to the number on the door to each room.
The numbered keys match the numbers on the corresponding room doors.
The keys are numbered to match the rooms.
You don’t need to tie yourself up in knots with more details.)