The most common and most likely answer to this crossword clue is the 5 plus 4 letter word SPEED DIAL
I've seen this crossword clue in The Times
S
P
E
E
D
D
I
A
L
More clues leading to the result SPEED DIAL
Buddy explains!

The straight part
We know that "Telephone function" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case telephone function = speed dial.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 SPEED DIAL mean?
SPEED DIAL verb- To dial using speed dial.