PS. The only time I used this on United was on an SFO-LHR flight about 10 years ago. I though it sounded quite informal. Eg. :-

ATC - 'UAXXX, cleared for take off. There's a ZZZ flight due to cross in front of you, so please get a move on'

UAXXX - 'UAXXX, we're rolling, we'll keep an eye out for him'


Cheers,
Rick