The hands free bluetooth and onstar performance on the XTS is a joke. My dad is ready to get rid of this thing and go back to a Buick Lacrosse. It has been in for service at the Cadillac dealer several times for this. They have blamed this on the Iphone and its settings. The tech hooked his Samsung up and it sounded exactly the same. Onstar has told my dad several times that they could not understand him and to call back. He actually called onstar service from his home phone and the guy listened to his Onstar calls - apparently they are digitally stored and he could not believe how bad the audio was. Just like you described - I think it is like the windows are down and you have your head and phone both out in the wind at highway speeds. Cadillac says that it works as it should so my friend at the Chevy dealer had a new microphone installed. Still bad. When the call is initially made or received the noise is so bad you can't hear anything but the "wind noise" but it gets better after a short time but still you can not carry on a conversation because the audio is so bad. When you stop the car you can. While talking to the cadillac tech when he was driving around I knew he was in traffic because when he stopped I could hear him clearly. There is an electrical problem that is causing this and I think that Cadillac does not want to admit this because the fix may be expensive. Dealerships are not to tell customers there is a problem unless there is a known fix for it. There must be some cars out there that do not have this problem or everyone would be complaining. It is not the phone because it does the same thing with Onstar. Before you buy a new or used Cadillac be sure to pair your phone and make a few calls and make sure you get a good one. I have never heard a blue tooth as bad as the one in my dad's XTS.
I have the identical vehicle as my sister does my voice quality is not great, however it is not nearly as bad as my sister's vehicles performance. It is as you have described. Same response, dealer service says it is working as designed.
I don't know if Cadillac will figure out or fix this problem but at least they are working on it. Dad was very persistent in his complaining and kept calling the dealer and Onstar. Onstar is really the ones that got the ball rolling from the complaints. They could not carry on a conversation with my dad through the regular onstar system since it uses the same microphone as the bluetooth. He has been in a demo XTS for over a week. It also has the same problem. The dealer said that there are factory guys involved now and they checked new cars on the lot and a lot of these have the same issue. Strange that Cadillac is the GM flagship loaded with all the fancy electronics and this system is so poor. I will update if this is ever fixed.
After nearly 3 weeks driving a demo with a poor quality blue tooth system the dealer called to say that the "GM Engineer" got my dad's car fixed. They supposedly added some foam around the microphone and additional insulation around the exhaust to eliminate road noise. This just took them short of 3 weeks to do this. Pretty efficient work. When I was called from the vehicle over the bluetooth I immediately asked if the windows were down. He said he was going 40 with the windows up. The "wind" noise started to lessen and I then said I think you are stopped at a light. I was correct. A quick call to Onstar - and the operator could not understand my dad. They called him back and then they could. There is not a pattern as to when this thing will work correctly. It works best when you are not moving. The "wind" noise is almost none existent at that point. I believe that the problem is something electrical as there is not a lot of road noise in the vehicle at 40 miles per hour. This particular car has the worst perfoming bluetooth system that I have ever heard. If you are moving at all it sounds like the windows are down.
His car has only 15,000 miles on it at this point so he will probably live with the problems until 25-30K when he normally trades off for a new one. He may go back to a Buick. Whatever he does he will make sure it has a functioning bluetooth before purchase. Cadillac has again told him that the system is working as designed. Something positive to think about though. My wifes friend has a large Mercedes SUV. My wife does not know the model but while talking to her friend her friend says - Sorry but I am about to get in my car - my blue tooth is horrible so I hope you can hear me. Cadillac has company in the poor blue tooth category for high end vehicles.
I know that not everyone is experiencing these problems but for those of us that are it us frustrating to purchase a new vehicle that is defective only to be told there is not an issue. This is common practice when there is a defect that is not present in every vehicle and the cause/solution is not known. If you plan to use the bluetooth system in any vehicle that you may purchase you may want to make sure that it works before you close the deal. This goes for any brand of car. Some cars are worse than others and I am sure that you get the same response from other manufacturers that the system is working as desgined if your car falls short in the audio quality.