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It is possibly a refurbished phone. Apple's terms and conditions allow them to give refurbishments in place of original phones where repair is not possible. Refurbished phones are often better than new as they are thoroughly tested and any suspect part replaced before hand over. The balance of your warranty is also covered.
Not absolutely sure but I seem to remember that an F initial letter is the code for refurbished.
In any event Apple are entitled to offer a refurbished phone. Check your terms and conditions.
Please understand, we here, are volunteer helpers. We do not answer on Apple's behalf; only Apple can do that. Our position is mainly to answer technical questions, though we do try to help in matters like this one.
Question: Different imei after replacement