You didn't say what phone you have so I can only speak for my iphone 7+ (or probably iphones in general since I don't think it is any different on the others). The volume control on the iphone is either for the ringer if there are no active apps on the screen that have a volume feature or volume for the currently active sound playing app - either local sound or streaming sound (which could be just audio or video with audio such as Youtube). When you pair an external sound playing device to the phone - the volumes of both are linked together. Changing one will change the other. Now there should be no reason in my opinion why if you lower either end, it doesn't lower on both at the same time.
The only way on an iphone at least to lower it on the phone or QC35 end and not the other is by unlinking the two.
Bose is going to ask you so it is probably a good idea to start at ground zero - remove all instances of the QC35 on your phone's bluetooth list, reset the list on your QC35 by holden the volume switch to the right for at least 10 seconds and then re-pair and try.
Thanks for posting. I didn't have access to an original QC35 but I was able to test this using a QC35 II and a Samsung S7 running the latest software and can confirm that when I adjusted the volume on the headset, it did change on the phone. So I believe they must be linked.
I will investigate further and post whatever I find here.
Tony G - Community Support