The most common and most likely answer to this crossword clue is the 4 plus 2 letter word DROP IN
I've seen this crossword clue in The Guardian
D
R
O
P
I
N
More clues leading to the result DROP IN
Buddy explains!

The straight part
We know that "Call" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case call = drop in.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DROP IN mean?
DROP IN noun- One who casually drops in.
- An informal event that does not require booking in advance.
- Provided for short-term use
- Fit to substitute some element in a complex system without changes to the existing infrastructure