I've been with T-Mobile for at least 5 years now, probably longer. I started with a phone from them, graduated to a Sony Ericsson clam shell that I could only get in Taiwan (figuring out how to change it to English display was fun), then a 650 and now a 680. I've been using the same SIM all along. I've never heard of this before but I would think there should be a way to ask the phone what kind of sim it's using (3G, not 3G, etc). Maybe once we know that, we can get several T-Mobile users on this list to say what type of SIM card they have.
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