When you call off something, you cancel it.
EG: The president was in the hospital, so we had to call off the meeting this month.
When you come across something or somebody, you find something or meet someone without expecting it.
EG: I came across an old photo I forgot I had while I was cleaning the attic.
When you come up with something, you think of a new idea.
EG: How are we going to solve this problem? Nothing seems to work! We need to come up with an entirely new idea.
When you come over, you visit somebody’s house or a place.
EG: We’re at Jake’s house. Would you like to come over?
When you cut out something, you omit it.
EG: He cut out smoking for two weeks and he feels great!
When you cut down on something (usually expenses), you try to reduce it; to lessen it.
EG: The company didn’t do very well this month, so we need to find ways to cut down on expenses.