https://www.cnet.com/news/some-pixel-3-users-are-experiencing-mic-and-call-quality-issues/

A few of these reports originated back in November and were escalated by a Pixel community member, according to Android Authority. Some solutions were proposed, including rebooting to Safe Mode and a factory reset, but they don't appear to fix the issues entirely. 

This isn't the first time Pixel phones had these kinds of issues. In 2017, hundreds of users complained about microphone problems with the first Pixel, which Google cited was due to a hardware issue. We don't know how many phones have been affected by this current problem, and I haven't personally experienced any of these problems during my months of having the Pixel 3. Still, this is a annoying problem for those who are experiencing it, and it's cause for concern if you're planning to buy a Pixel 3.