Adam -- postpaid T-Mobile and AT&T subscribers can roam for free, and pretty seamlessly, on each others' network in places when only the other carrier is present. But look at their prepaid service maps, and you'll see their networks are very separate.
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About the "sometimes clever" commercials -- Chad has already moved on to pitching chicken: http://www.youtube.com/watch?v=EKr_bvd8h2I
Adam -- postpaid T-Mobile and AT&T subscribers can roam for free, and pretty seamlessly, on each others' network in places when only the other carrier is present. But look at their prepaid service maps, and you'll see their networks are very separate.