MXP Ground: XXX123, standby for push and start clearance.
My question is: Do I have to readback anything for the above transmission? or simply wait for ATC to contact me again?
Is it true that it is incorrect to reply "standing-by" every time a controller says standby?
Basically my Captain seemed very irritated that I didn't give a reply to the above trasnmition, but my understanding is that ATC has neither approved or dennied anything and that I should wait for them to contact me.
Thanks for contribution.