FIXED.
It's a problem with your adapter cable. I tried several, and they would all play music, they just won't work with phone calls, and I finally found a post by an electrician on some obscure website that explained that problem. It has something to do with most auxiliary cables only supporting 3 pins, vice 4, but I don't think you really care about that. The cable I am using to get my Iphone 12 Pro Max to use my headphones (or in my case my car speakers) is from Amazon: UGREEN Headphones Adapter for Apple MFi Certified iPhone Lightning to 3.5mm Jack Converter for iPhone 12, but there are several that work, that's just the first one where I found the certification difference.