An original creditor is not normally a debt collector under the FDCPA, but there is one exception.
FDCPA 803(6) states:
"the term includes any credtor who, in the process of collection their own debts, uses any name other than their own which would indicate that a third party is collecting or attempting to collect such debts."
If they use a name in their contacts that would lead to the consumer to believe they are a party collecting on behalf of the creditor, an OC can become a debt collector under the statute.
If they are not a debt collector, then you dont have recourse to the federal FDCPA regarding the collection practices, including a cease communicatin letter or action for violation of prohibited collection practices, but most states have their own debt collection practices statutes, some of which apply to both creditors an debt collectors. You need to check for basis for civil action on state, not federal, statute.