To phone us about nothing is cruel (7)
I believe the answer is:
callous
'cruel' is the definition.
(I've seen this before)
'to phone us about nothing' is the wordplay.
'to phone' becomes 'call' ('call' can be a synonym of 'phone').
'about' is a charade indicator (letters next to each other).
'nothing' becomes 'O' (O resembles 0).
'us' after 'o' is 'ous'.
'call'+'ous'='CALLOUS'
'is' acts as a link.
(Other definitions for callous that I've seen before include "Emotionally hardened" , "Cruel or unsympathetic" , "Heartless" , "Cruel and unfeeling" , "Insensitive, cruel" .)