Why "I will call you back" instead of "I will call back you"? Here "call back" is a phrasal verb, and "back" is a particle. Then why are we separating the particle?
Is there any rule for it?
The variability of the position of the particle is, I suppose, conventional. I am not aware of any rule for it.
'I will make up the bed' converts to 'I will make the bed up'. Or 'I will close down the business' can be 'I will close the business down'. As regards 'back': 'We will get back the deposit' could equally be 'we will get the deposit back'.
But 'We walked down the road' can never be 'We walked the road down'. In the same way 'I will call you back' cannot be 'I will call back you'.
There are multitudes of these which it might be worth exploring.