Some questions:
1. What state are you in? (you should always select your state when posting a question - it's nearly always relevant)
2. How did you receive the notice (mail, email, SMS etc)?
3. What website did you use to check the infringment number?
4. What phone number did you call to check the infringment number?
There are a lot of scams going around regarding fake infringment notices. It's happening in all states and at the Federal level as well. Some try to get you to pay money while others are just viruses (usually trojans).
If the infringement number doesn't check out, then it's possibly one of these scams.