Anyone using this phone with AT&T in the US?

I love this phone....there, I said it. But it's driving me crazy.

I bought it from a Chinese retailer, who shipped it with X821 firmware, but the box states that it's an X820. I doubt there is any difference in the hardware.

I would have left everything as it was, but I can't seem to make phone calls (don't have a data plan yet...yeah, I know....) with anything other than 2G, and that is about to go extinct. When I set it to WCDMA, it shows a UMTS connection, with no carrier designation, but the tower is the same one I connect to with 2G. The phone does sometimes show an AT&T 3G connection in the status bar, but I can't make calls, even though I have full signal. When I place the same SIM in my old HTC Inspire, the same monitoring app shows "UMTS - AT&T HSDPA+" and works just fine.

I'm at my wits end. I have rooted, installed TeamSpain recovery, replaced the firmware with the X829 and even tinkered with the frequency configurations through DFS Studio (which did a lot of complaining, and I'm still not sure if the changes stuck 100%). I also played with APN settings and tried the SIM in different slots, both alone, and with another SIM. And after all of that, still no progress in getting this phone to use a 3G or 4G connection to make calls. If I don't set it to "GSM only", no phone calls in or out. I'm essentially right under a tower, so signal shouldn't be an issue.

If anyone is using the same provider in the US, and is not having these kinds of issues, I'd really appreciate hearing what firmware you are using, and/or what you did to get things working.

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