Don't call them; knowledge of USCIS customer representatives who take calls is no better than ppl in this board. Unless you can show me an official web link from uscis.gov, then I'm telling you that you don't need to enroll at all if your COS application is submitted before your F-1 status expires ( which is the last day of registration), AFFIRMATIVE. But to be safe, keep a record from USPS or the delivery service showing that the application is received by USCIS (like a delivery confirmation) as a proof that the application is submitted in time, in case something went wrong and you didn' t receive the receipt.