When I see the word "cosign" in the message subject, I cringe. I know that the story is not going to have a happy ending.
When you cosigned you took on full responsibility for payment, regardless of what your friend did and regardless of what you knew or didn't know.
If you've already paid the collection, you've lost any bargaining power you had to negotiate a PFD.
But anything is worth a try. Give it a shot -- write to the cell phone company.
Good luck, and let us know what happens!