call on
call upon
drop in on
To make a call upon; visit.
Mr. Brown called on an old friend while he was in the city.
Jose didn't know the answer when the teacher called on him.
Last night several friends called on us at our home.
To ask for help; to ask for a response from.
He called on a friend to give him money for the busfare to his home.
Why don't we drop in on Sally a little later?
drop in the bucket
A relatively small amount; a small part of the whole.
Our university needs several million dollars for its building renovation project; $50,000 is a mere drop in the bucket.
drop in
To make a short or unplanned visit; pay a call. Often used with on.
We were just sitting down to dinner when Uncle Willie dropped in.
The Smiths dropped in on some old friends on their vacation trip to New York.
drop into
To stop (somewhere) for a short visit or a short time.
We dropped by the club to see if Bill was there, but he wasn't.
I dropped into the drugstore for some toothpaste and a magazine.