I experienced this last weekend. I went to northern Wisconsin and caught off guard when my iPhone suddently showed AT&T in stead of T-Mobile at the to so I can confrim this has happened to me.
They roam on AT&T in some areas but they don't have roaming access to the entire AT&T network footprint. My phone switches to AT&T in some parts of northern SC when traveling to Myrtle Beach but in some remote areas of Eastern NC where AT&T has voice and Edge service, Tmobile goes to 'no service'. Also roaming data is capped at 50 or 100MB per billing cycle depending on your plan and the speeds are severely throttled even when it shows AT&T "4G".
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