David Patrick Kelly has already answered this question in a prior interview. He said that the director never really gave him insight as to who he was supposed to be talking to on the phone but he felt personally that it was someone that Luther looked up to like a mentor of sorts.

His probation officer.

Remember when Luther called he said Cyrus had an "accident" so this line would suggest he acted on his own and whoever he was talking to was not in on him killing Cyrus.
I always thought that he said that with a bit of sarcasm or in a manner of like "it's done" without actually saying it and the person he was talking to knew exactly what he had done.

Remember when Luther called he said Cyrus had an "accident" so this line would suggest he acted on his own and whoever he was talking to was not in on him killing Cyrus. But I always thought it was hilarious that even the Rogues checked in to let the rest of their guys know they were ok! The Warriors never even mentioned any of their other gang members or even hinted at finding a phone! Lol!

This topic has been discussed before on the board. It is a great mystery of the film.
I do not think he was talking to the cops as many have suggested. I simply believe that Luther is crazy and one of those people who wants to watch the world burn. When asked by the Warriors why did you shoot Cyrus he answers "no reason." At that point he had a gun in his hand and was about to kill Swan, so there is no reason to believe he was not being honest.
I think he was checking in with his gang's headquarters.

Some people thing he was talking to the Italian Mafia, which makes a lot of sense if you think about it.

I always thought Luther was working for the cops.
There's only birth,copulation and death. Everything else is guess work!