I have a similar problem. I can use data no problem but can't make phone calls (I even have T-Mo's h&h add on). I chatted with customer support and he suggested several things but none worked. One idea worked for about one hour and k'boom...my 8320 started dropping calls again. I can make calls via wi-fi in my local coffee shop, nevertheless.
If the three of us are having the same issue then.... Ouch...
(I have the Airport Extreme "n" router)