iPhone 4S doesn't reliably use chosen audio input if paired with a bluetooth headset
After pairing a bluetooth headset with my iPhone 4S, my iPhone does not consistently use my chosen audio device when making calls to other people. Does not seem to be a problem when checking my voicemail.
Examples:
1. With my headset on (and showing a connection to my iPhone), I'll make a call. My iPhone will initially show my headset as the chosen audio device. Moments later, the speaker is automatically chosen, and tapping the button to switch back to the headset is ignored. Sometimes I can tap the button to switch to the iPhone, but the audio often gets lost (not on the iPhone, the speaker, or the headset, but the call is still connected).
2. With my headset on, I'll make a call with my iPhone chosen as the audio device. Despite showing the iPhone as the chosen device, the call actually uses the speaker. Attempting to choose the iPhone results in lost audio, as in the above example.
I have only seen this problem on my iPhone 4S. I have seen it with two different headsets (Motorola H270, Jawbone Icon), one of which (Icon) has never been paired with another phone. My wife's iPhone 4 running iOS 5 does not exhibit this problem. Turning my 4S off and on again seems to help for a while (less than a day). Removing all headset pairings from my 4S seems to resolve the issue, which is really not a solution since a hands-free device is legally required when driving.
I've tried backing up and restoring my iPhone and it seems to be working properly for now. In a day or two, I should know if it really is fixed.
iMac, Mac OS X (10.7.2), 3.4GHz Core i7 (SNB)