"He will call you back immediately."
"回" and "还" in Chinese both syntactically has meaning of "return, go back, revert". Which one to use exactly depends on the context and it is a matter of practice. In your case, "he will return your phone immediately" can be translated as "他会马上还你的电话", word by word, however authentically we will say "他会马上还你电话" instead, or "他会马上把你的电话还给你", which can be literally translated as "He will return your phone to you immediately".
Also, in my experience, 回 =return to somewhere (i.e. home, work, etc.) Whereas 还/还给 = return object to person. Hope this helps :)
Yes, you're right! There is also 归回原位 (return (back) to original place/position) which can apply to people or things.