My experience corresponds with yours, though you've repeated the experience more often. I find the reverse is true, also - if I need to disconnect my headset in the middle of a call because of poor performance, the call drops.
I couldn't find any reference to the issue in the iPhone support area at Apple. A similar problem is reported at the FAQ link below, where users report the speaker won't work once the headset is disconnected (I can find nothing where the problem results when connected, as in your case.)
At this time, similar problems seem to be reported often enough to lead me to think this is a software/firmware problem. Keep the phone current on its operating system, and maybe the problem will resolve itself.
If you really can't live with the issue, though, maybe it's time to take the phone to an Apple Store for their take on the problem.
Good luck - and report back so we can all learn from the problem.
Source(s):
http://www.iphonefaq.org/archives/97379

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