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The standard repair process doesnt actually test the operation of the mass memory. This is why the N97 I received (which was 6 months old - not 7 days!) still had photos and videos from the previous owner. In this particular case the mass memory was write protected and hence could not be erased or new info written to it. Since the repair check list is supplied with the repaired phone I can say that the testing performed at the repair centre is inadequate. Hopefully things have improved.
I seem to recall that the GPS, Mass memory, FM transmitter, Camera, Wifi etc were not tested at repair. The only things that were tested were screen, keys, make a test call, fit new bezel, check for scratches, etc...i.e. all the techie bits were not tested - only the basics are covered.
As you got your phone from a 3rd party the warranty and repairs will be via them and/or the manufacturer.
If this was with us you would have a normal repair and is very rare that an exchange will be done straight away.