I called Crutchfield and Alpine this evening and told them about a similar problem I’m having. This might be what you are describing, but through Bluetooth or USB the ringing volume is fine, I can hear the caller fine, but I’m very quiet. I believe sometimes Siri has trouble hearing me also. I placed several test calls to myself adjusting the mic volume in the head unit but it doesn’t seem to make a difference.
I also have an Alpine ILX650. I was using a Samsung Galaxy S7 before upgrading to an IPhone12 Pro. I haven’t moved the mic or changed the install in my vehicle at all except for the usb cable.
If you have a solution please let me know, I can’t make or receive calls currently through CarPlay because I can barely be heard even with the vehicle in park.