Here is the bug report that I just submitted to Apple. I thought I'd post it here as well. Add me to the list of frustrated Bluetooth users.
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Ever since I upgraded to an iPhone 4 (the second day it was available), my Bluetooth earpiece has had intermittent connectivity problems, which never happened with my 3G. It will lose and regain connection with the phone seemingly at random. It will act as if I have wandered too far from the phone, but it is always with me at the time (in my pocket, on the car dock in my car, etc).
Here is what I've tried to remedy the issue, roughly in the order of my attempt:
* I always use the most up to date iOS software
* Using a different Bluetooth headset
* Restoring my phone from a backup
* Restoring my phone as a new phone
* Using my Bluetooth headset with my old iPhone 3G (there are no problems using my iPhone 3G)
* Getting a new (refurb) iPhone 4 from the Apple Store
* Removing the sim card, in case there was some interference between 3G & Bluetooth
* Performing a "Reset Network Settings" (via Settings App>General>Reset>Reset Network Settings)
* Manually telling the phone to "Forget" my Bluetooth headset, and re-pairing it afterwards
Here is the pertinent hardware that I've used
* 2 iPhone 4's, 32GB (the first one was bought the second day it was available, then swapped at the Genius Bar on 27-Nov-2010)
* Jawbone Prime headset (my primary headset)
* Jawbone Icon headset
Here are some observations I've made regarding the behavior of the problem:
* It usually drops connection when leaving a WiFi network that I was connected to. For example, as I drive away from my house and leave my home WiFi range, the Bluetooth connection will drop as the phone switches over from WiFi to 3G. This is not the only time it drops, but I have seen it do this many times.
* There seems to be some geographical consistency in when the Bluetooth drops. For example, it almost always drops when I go to my mailbox in front of my house with my headset on my ear and phone in my pocket. This could be related to the previous observation.
* Some of my attempts to remedy the issue have had some short lived success. Restoring the phone, getting a new iPhone 4, and resetting network settings have each made the problem go away, but only for a while. After getting a new iPhone 4, the problem went away for nearly 3 weeks, but started to happen again after that.
* It does not seem to matter whether or not I'm on a call when the Bluetooth drops. If I am not on a call, my headset will beep indicating loss of connection, and the Bluetooth logo on the iPhone will blink and turn off, indicating loss of connection. If I am on a call, both of those other things happen and my conversation will switch over to the handset and away from the headset.
* Sometimes while using my headset during a call, the other party complains that I sound bad (some have said there is "digital noise"), but I have never had a sound quality problem in hearing the other party.
* The fact that I have tried different Bluetooth headsets and different phones leads me to believe that this is either a problem with the hardware design itself (I sure hope not!), or with the software, as those are the only things that are common to everything I have tried to fix this problem.
bj