I called in my trouble report to AT&T a few days ago, and today I was surprised to hear from a tech support person. She asked if I had used the MicroCell lately, and I said I hadn't because it was pretty much useless. She told me in the last couple of days a 'script' had been sent to my unit, and asked me to try again. Since she called me on my iPhone I walked around the house, and quite a distance out in the yard to test it. The call was fine. I've used my iPhone several times today, all with good results.
I can't say it is a permanent fix, but whatever they did, it has worked for me so far. If you've had MicroCell problems I'd strongly suggest you file a trouble report, and if you have, be sure to follow up with AT&T. The tech support number that deals with MicroCell issues is 877-998-5175. Ask them to download the mysterious 'script' to your MicroCell and hope for the best.
I had thought this was completely an iPhone 4 issue, but the tech I talked to said there are issues on some other phones as well. The jury is still out on whether this is a real solution, but at least in my case I haven't heard any bad audio since AT&T downloaded the fix to my MicroCell.
Let us know your experiences, good or bad!