I am having the same problem. I returned my iPhone 8 Plus 64GB Space Grey yesterday after speaking with tech support for over 2 hours. The replacement phone I got yesterday is doing the same thing. Cracking noises during calls. If this is not resolved stat, I will be returning for a refund and going back to my iPhone 7.
It won't be resolved any time soon I imagine. I'll be returning my phones too. I'll wait for them to fix it however and just buy them later after I buy the X and other phones