I contacted them via chat today about this, and they told me that no one had called about this issue before. I gave them the URL of this chat. They asked me to upgrade from 15.1 to 15.2, check if my case covered any of the small holes, asked if it happens in multiple apps, etc. My family uses Whatsapp, and it worked fine on my old iPhone 8, on my husband's iPhone SE, and one an old LG Android Phone. However, any of us that have an iPhone 13, when we talk, the person on the other end hears us on and off unless it's TOTALLY quiet on their end (which it never is.) On prior devices, the solution would be to turn on noise cancellation, but now that appears to be gone. So, as many of us as have 30+ minutes to spend with tech support should call this in. Insist that they escalate it to engineering.