The recent crash over Switzerland shows what happens when pilots obey ATC instead of TCAS.
If its fitted we must obey TCAS and after we have called TCAS climb/decend thats it. ATC just have to fillout the paper work of why the 2 planes got that close. And try and work out a solution what to do with all the hardware after it has completly cocked up their cunning plan.
TA are issued all the time in the UK thanks to most light aircraft being on tatical transponder A squawks so they can't be caught for level busts which is great joy at Fl200 +
From menory it is a timing thing and its something scary like 30secs before impact when you get the warning.
And the seperation again from memory is 300ft.
MJ