The most common and most likely answer to this crossword clue is the 3 letter word APP
I've seen this crossword clue in The Times
A
P
P
More clues leading to the result APP
Buddy explains!
The straight part
We know that "one may be on the phone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one may be on the phone = app.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 APP mean?
APP noun- An application (program), especially a small one designed for a mobile device.
- Application (use, purpose; not a computer program)
- Application (to a college etc.)
- Appetizer
- An appearance in a game (e.g., a player with 10 apps in a season played 10 times)