When does trail assume sideline responsibilities on a transition?

[Nevadaref]

The new Lead will still have that part of the sideline which is in front of him as he moves down the court. The Trail will not have a great look at such a play, so it makes sense for the Lead to take the call. The Trail has everything that is behind the new Lead.

The new Trail assumes this responsibility as soon as possession changes and heads the other way or the ball becomes dead and a throw-in for the team going the other way will ensue.

Of course, there certainly are times when the new Lead should be ready to help with a sideline call.