My ex girlfriend report my iPhone stolen

Who's name was the bill in? If it was in her name, there's not much you can do. The phone belongs to her. It doesn't matter if you were giving her money toward the bill or not. If it was in your name and she did put it in the system as stolen, then you need to contact her manager and possibly the police.

Apple doesn't have a lost/stolen list. If she said she did this, she's probably pulling your leg. If your carrier has such a list, then you need to check with them.

Contact the Sprint store's manager or if that person is of no help, keep going higher in the Sprint organization. No one at Apple is involved in this.