Bluetooth Audio on Headset for Calls is Bad

Hello!

I built my own headphones based on the BT401 (link), and they sound great!

I can play any audio like music and videos from my iPhone 15 Pro to them, and it is crisp and clear, and I am able to adjust the volume correctly.

The problem is that for Siri, or phone calls, the audio is terrible! The volume stays all the way up no matter what I do. I also checked all the settings I could find including lowering the ringer volume, but nothing works. Also, the audio is extremely fuzzy, and hard to hear, despite it being so loud. I have also found if I initiate a call from my headphones, the mic will not work.

My best guess is that something is wrong with the bluetooth profiles, but I am not sure and have no clue how to fix it!

What is going on here? I've tried everything I can think of and this is getting very frustrating. Any help here would be greatly appreciated!

Edit:

Just to make it clear, I am using a iPhone 15 Pro connected to a BT401, and my iPhone is fully updated to the newest iOS of 18.3 (although I have also tried iOS 17 and 18.1, 18.2).