UC520 configuration : hook off

Hi,
I actually deploy a uc520 solution. I have 2 T0 and the four lines have the same number, 3 softphone and a IP Phone 7961.
During a call, when the personne i talk hooks off, i don't ear anything. So if i don't look at my ip-phone, i don't know that he hooks off!
Is there a command line to have the "hook-off tonality" when the call is stopped ?

Thanks