Verizon's "can you hear me now?" guy has retired from that gig, but it now seems people receiving calls from an iPhone 4S are picking up the torch and asking the same question.
That's because there seems to be a bug--reported on all three major iPhone carriers in the U.S.--that's occasionally keeping owners of the new iPhone from hearing anything in their earpiece when making outgoing calls.
At last check, the discussion of the issue on Apple's support forums is 29 pages long and the current theory within the thread seems to be that a driver issue is to blame. Users have identified a number of workarounds, including toggling the speakerphone on and off, plugging and unplugging headphones, or activating Siri.
I've contacted Apple for a response, but haven't heard back. Developers have already been fed an update to iOS 5. We'll see if this issue miraculously disappears when the update rolls out to regular users in a few weeks.