First of all..the term is Cease and Desist. Orders such as this rarely, if at all, arrive over the phone. It is a written order usually issued by a judge, at the request on a individual or business. This sounds fishy but without details, I can't say more.
Secondly, fan fiction guidelines can be tricky. If you are posting this fan fiction, remember, even though it is YOUR writing, you are working under the original authors name and if you are in anyway adding information that is detrimental to the original story, you could be held libel. Yes, this means you could face a law suit if you are using any type of slander, defamation, etc. (If you are unclear of these definitions, look them up. Dictionary.com can help you with that. If you have violated any of the fan fiction terms/guidelines you could be fined. Writing for you own pleasure is fine but if you post these stories on the internet, they then become public (exposed, as you stated). If you write them only for you, say in a document, to print, keep, etc. that is fine.
There is a ton of fan fiction on line and this is not illegal, just follow the guidelines and you should not have a problem.