after service, cannot register my phone

Last December, just before the guarantee was over, and since the battery of my iphone 5 had issues, I took my iphone to an Apple service store, in order to replace the battery. As a usual practice, Apple replaced my phone with a refurbished one, having itself a different serial number. I tried to register the replacement phone, but wont register, because, it says, is already registered to a different apple id. bear in mind that the phone is activated completely with my apple id. How can I fix this issue?

Thanks