i’ve been using them with an iphone 15 running the latest os and i’ve never had ay problems with volume, only connectivity, sometimes it doesnt connect first time when you ake them out of the case and i have to put them back in their case and open again but thats the only problem. My best guess is that iphone 6 is running an older bluetooth version, you shouldn’t have any problems. Also, I don’t have the connectivity issues anymore with ios 18 and never had any other problems with other devices, i use an aptx bluetooth adapter for PS5 and it never fails to connect. hope this helps 🙂