Ring any number and keep waiting to be connected (2,4)
I believe the answer is:
on hold
'waiting to be connected' is the definition.
(I've seen this before)
'ring any number and keep' is the wordplay.
'ring' becomes 'o'.
'any number' becomes 'n' (I've seen this in other clues).
'and' is a charade indicator (letters next to each other).
'keep' becomes 'hold' (holding is a kind of keeping).
'o'+'n'+'hold'='ON HOLD'
(Other definitions for on hold that I've seen before include "Postoned, or awaiting reply to phone call" , "waiting for reply" , "Left pending" , "In a state of postponement" , "not through yet?" .)