47 CFR §1.718

When an informal complaint has not been satisfied pursuant to § 1.717, the complainant may file a formal complaint with this Commission in the form specified in § 1.721. Such filing will be deemed to relate back to the filing date of the informal complaint: Provided, That the formal complaint: Is filed within 6 months from the date of the carrier's response, or if no response has been filed, within 6 months of the due date for the response; makes reference to the date of the informal complaint, and is based on the same cause of action as the informal complaint. If no formal complaint is filed within the 6-month period, the informal complaint proceeding will be closed.