FN is on the acknowledge letter that DIAC sends you. That indicates your application is officially in their database. Currently it'll take 4-5 months to receive the FN. It's a bit like 103. Once you get the queue number, then you can arrange whatever you want.
Don't just call them. Meanwhile also email DIAC stating your situation and see what they say. In that way, you at least have written confirmation in case what will happen in the future as a proof.