The problem with my new MPx220 microphone not working started driving me nuts because it was happening on every call. To me, having to switch to the speaker phone was not an acceptable work-around to a $300 phone not having a working speaker! (I also found-out that the caller on the other end gets a really loud "beep" sound when my phone is switched from the microphone to the speakerphone.) Before my 30 day return policy was up, I took the phone back and exchanged it. So far, my new phone hasn't had this microphone problem, but I have it configured differently so I'm not sure if the old phone was really broken?
With the new phone, I configured ActiveSync to sync with my company's Exchange server on 4 hour intervals instead of having the phone sync every 10 minutes. I don't use my phone to receive emails, I just like the Exchange calendar sync feature, so I don't have any real need to have the phone constantly syncing.
I feel that by having the phone sync every 10 minutes, there is a pretty good chance that when a call comes-in or when I make a call that the phone may be in the middle of performing a sync in the background and creates a problem that prevents the microphone from working. Another idea regarding the cause of the problem is that since the phone is running the Windows Mobile OS, perhaps there is a memory leak somewhere and after a full day of making calls to the Exchange server, things start malfunctioning and must be fixed by a reboot (the solution some propose to *solve* this problem.) Rebooting seemed to be a good way to fix issues in previous versions of Windows!
One other advantage I've found to setting the sync interval to 4 hours is that any speakers that are near the phone don't pick-up the signal that the phone emits when it's attempting a sync. Our Cisco Avvid phones at work buzz and hum all day long if the MPx220 is syncing a lot, and that got really annoying, not to mention that it just seems like a health risk!