Have you found yourself making a call with your new iPhone 4S only to all of a sudden notice it sounds like your call is being routed via cavernous tunnel? A number of users have been reporting problems with echo during voice calls, and if the same thing has been bugging you, there’s a quick work-around for the issue.
Key to the occurrence of this echo problem seems to be the use of a wired headset. While a call over such a headset starts out fine, sometime further into the call (it can take seconds or minutes) the echo kicks in.
The good news is that all signs point to this being a software problem, so an official fix should be possible in a future iOS update. We can only speculate at the the precise cause, but it sounds like a problem keeping audio buffers properly synched-up, causing a growing delay that leads to an echo after some time.
For now, the simplest thing to do is wait until you notice the problem, then quickly engage, then disable the speakerphone. Doing so will reset things, restoring your audio quality, but the echo will likely creep back some time later.
There’s been no official comment from Apple regarding the reports.