At risk when telephoning (2,3,4)

I believe the answer is:

on the line

I'm a little stuck... Click here to teach me more about this clue!

'at risk' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for on the line that I've seen before include "when one is at risk" , "Caller may be" , "Using the phone" , "At risk - or speaking on telephone" , "At serious risk" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app