Hello GBFL2021.
Thank you for using Apple Support Communities. You mentioned that you're experiencing issues with phone call sound quality when using Bluetooth headphones with your iPhone. Specifically, you're hearing an echo. We can understand how this would be an issue. Allow us to investigate this issue and assist in any way we can.
You also stated that you've purchased a new headset and the issue continues occur. Testing a different head set was a great step to try. If you connect these headsets to a different iPhone, does the echo continue to occur or does it only occur when using your iPhone 12 mini? If this issue occurs on another iPhone, try checking if your headset requires a firmware update. You might have to reach out to the manufactures for steps on how to do so.
However, if the issue only occurs with your iPhon 12 mini, and you're already full updated, and basic steps such as restarting or re-paring your headset doesn't help, reach out to Apple Support. They might have to investigate this issue further with you. Just in case, you can use the link below to reach them:
Get Support - Apple
We hope this information is useful. Cheers.