No, they're not the same thing.
Will be able to obviously talks about a future event, while can talks about the present.
I can swim, so we should go to the
pool.
Means I can swim already, I learned it before and I'm capable of doing it now.
I will be able to swim sometime in
the future, as long as I take swimming
lessons.
Means I cannot swim right now, but I'm hoping to learn in the future.
Probably you meant to ask the difference between can and am able to. If so, there is a difference, yes. Let me explain:
I am able to swim, but I can't, because my skin is damaged.
This means that I am capable of swimming, but I am prohibited to do so, because of an infection. So to put it to an explanation:
To be able to talks about an ability to do something, but it does not mean you are allowed to do such.
Can talks about something you are allowed to do, or about your ability to do it. This is therefore ambiguous.
It is alright to use against after do. Against simply means in opposition to, so
The USA never did anything against violence
means
The USA never did anything in opposition to violence.
On the other hand about means with regard to, so
The USA never did anything about violence
means
The USA never did anything with regard to violence.
Best Answer
Yes. The 'to' is part of an implied infinitive, meaning "I will call you back as soon as I am able to call you back". If context is understood (in this case, you have already stated that the intended action is to call back), the preposition is not necessary.