The most common and most likely answer to this crossword clue is the 6 plus 4 plus 3 plus 4 letter word PLEASE HOLD THE LINE
I've seen this crossword clue in The Guardian
P
L
E
A
S
E
H
O
L
D
T
H
E
L
I
N
E
Buddy explains!
The straight part
We know that "Request to caller" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case request to caller = please hold the line.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.