Refurbished Phone

I handed in my phone for repairs on the 19th of march 2015 as my phone was still under the 1 year warranty. <br> There was a problem with the audio not playing , when plugging in any headset.<br> <br> After two weeks I received my phone back on the 2nd of April. I just made it for collection as they were closing for the long weekend. I put my battery in my phone straight away and left. A week later I bought a 16gb memory card and when I removed the battery I noticed a refurbished sticker had been placed on my phone.<br> <br> I checked serial numbers on my phone to numbers on the box it came in and they do not match. <br> <br> My question is, did I get my phone replaced with another second hand phone ? Is this allowed ?. If my phone still falls within a warranty. Am I not suppose to receive a new unit, if the phone I sent in could not be fixed ?<br> <br> Thank you