Half the utility of a smartphone is obvious from its very name. At a minimum, it should be able to make calls, which requires a working mic and speaker. For some, half of that simple equation is missing when it comes to the Pixel 2 and its microphone. When making calls, it just doesn't work. The current workaround is straight out of an NES-equipped childhood: blow into it.
The problem is straightforward. When using the bottom mic in phone calls, when using voice recognition, etc., instead of recording sound, it's just silent. There's a thread going related to the issue on the Google Product Forums. Most of those with the problem have the smaller Pixel 2, though there are a few reports from those with the Pixel 2 XL.
While there's no permanent solution just yet, some have claimed it can be temporarily mitigated by blowing into the bottom speaker. An example of both the problem and its unofficial fix can be seen here (warning, turn down your volume!).
The cause is yet to be determined, and the same person who found this interesting workaround claims that repeating the action breaks the mic again. In either case, others have found a permanent fix in the form of an RMA, as not all units seem to have the issue.