Allowed to phone up? (3)
I believe the answer is:
let
'allowed' is the definition.
(I've seen this before)
'phone up?' is the wordplay.
'phone' becomes 'tel'.
'up?' says the letters should be written backwards (in down clue: letters go upwards).
'tel' backwards is 'LET'.
'to' acts as a link.
(Other definitions for let that I've seen before include "Reason for scrubbing service" , "Was inclined to ignore" , "Umpire's decision" , "Award (a contract)" , "Permitted" .)