Well.....I'm glad you posted. And boy did I recognize my "former" self in your behavior!
I would have become involved, triangulated, and sided with my son 100%......but that was then and this is now.
As others have pointed out, the crux of the problem was that your son left the phone on the curb. If my son had done the same thing, he would have given me half truths and flat out lied about what happened in order to divert the blame away from him--which is where it should have stayed. And being the codie that I was (and I stuggle with that daily), I would have believed him and taken up his cause!
I got in the middle of so many of his relationships (including his relationship with my husband and his sister!) 20/20 hindsight is powerful vision.
Any time you accept and handle a problem that belongs to your son, you tell him by your actions that he is incapable of dealing with them himself. You insult his intelligence and cripple him. I know this because I did it.......and continued to do it well into my son's 20's.
Step away. Allow him to know that he IS capable and intelligent and able to solve his own problems. I sooooooooo wish that I had the resource (and knowledge) that you have and that I reached out for help. I didn't KNOW I was co-dependent. I didn't KNOW that I was doing anything wrong (even though others tried to tell me). I kept doing it......over and over and over again to my son's detriment.
It's hard to be a mom. It's hard to let our son's make mistakes. It's hard to watch them hurt. And it's hard to let them suffer the consequences of their actions. It's much much easier (or at least it was for me...or so I thought) to just deal with it for them.
gentle hugs from one Mom to another