Go to the Play Store and download High Pitch Blaster.
Next, download the app called Earpiece.
In Earpiece, up the max volume output to 100% past normal.
Route the sound output to your earpiece.
Go to High Pitch Blaster to turn on the app, and modulate the pitch up and down, hitting them all at different volumes, even going up to or just below 100% via the Earpiece app.
Next, do the same thing with some loud music.
Repeat until your calls are mostly or entirely free of crackling.
Freeze the apps or uninstall them, as they might mess with a setting or two, later.
I don't know how it worked, but it fixed the crackling in my earpiece. Uninstall or freeze the apps, and then restart your phone.
Let me know how it goes.