Translation:He will immediately return your call.
The developers should review those questions with lots of comments and feedback. It's equally natural to say "he will return your call immediately"
Apparently, one would use "歸還", "還給", or "退還", which all mean "to return" in the sense of "to give back". This sentence uses "回", which means "to return" in the sense of "to reply to".
Agreed. 回 is more of a return to a destination/reply to something. To return an object to someone is 还.
can someone explain the 的 here? is this literally "the phone (call) of him who will immediately return (it) to you"
你的电话 - your phone call, which is implied for him to have a call to return.
It are many uses, as you know, but I like the simple layout of this site: http://www.ctcfl.ox.ac.uk/lang%20work/grammar%20database/DeHandout.htm