Serial number question

I purchased new iPhone 15 Pro and after 3 months I received a problem in the Bluetooth and when I call they try to fixed but at the end they told me that we will send you new phone as I am at warranty and also have apple care but now I just checked and found that the serial number begin with T6 not like the original serial number M so what does that means and do I connect with them to returned or I continue with that without any problem thanks