U think the high speed networks are not CDMA, UMTS, WCDMA, HSDPA in the US?
Sprint and Verizon are CDMA, EvDo Rev A. T-Mobile and AT&T are GPRS/UMTS. Verizon is rolling out WCDMA (true 4G) and AT&T is planning HSDPA (3.5G).
However, the signaling and the protocols are not the only issues. The US and Europe also use different radio frequencies, two each, layer 1. Most GSM phones, including the iPhone, but not all, support all four frequencies ("World" phones).
The GSM agreement requires that all GSM operators support any GSM phone that supports their frequency and that shows up on their network. Therefore, if I bring my iPhone to Europe, Orange will connect me but they will bill AT&T for all service at a rate agreed upon between them.
When I show up on Orange or O2, they send a registration message to AT&T that says "I've got him, is he your legitimate use and will you pay us for his calls. If so send all calls for him to us."
In AT&T's case, I must have agreed with them in advance of going to Europe. Similarly, if you bring your iPhone to the US, either T-Mo or AT&T will serivce you and charge your provider.
So far so good. However, here in the US we have a scheme under which one may buy a phone form a carrier at a price subsidized by the carrier, in return for signing a contract of specified duration with the carrier. In this case, the phone will use a SIM from that carrier and the phone will be "locked" such that it will only recognize a SIM from that carrier. My understanding is that most iPhones sold in Europe are unlocked and will use whatever SIM is placed in them Thus, when you bring your iPhone to the US, you may use the SIM that you brought with you, or a SIM from either T+mo or AT&T. However, when I take my phone to Europe I may use only my AT&T SIM.
For most of the phones that AT&T sells they will "unlock" the phone at the end of the contract or, at their discretion, upon request. AFAIK, they have not unlocked any iPhones. Of course, since the "lock" is software, there are hacks.
Therefore, with "dual SIM" capability when you come here, you can continue to use your European SIM and also use a US SIM. However, since my phone is locked to AT&T, a dual SIM capability will not help me.
Another way to think of it is that if one can switch SIMs, the dual SIM capability is useful. My understanding is that modt iPhones sold in Europe are unlocked and will use whatever SIM is place in them However, if one cannot switch SIMs (Most AT&T customers), then "dual SIM" is not useful.
Therefore, the answer to the question that started the thread is that a "dual SIM" capability for the iPhone will work well with European purchased iPhones but only "unlocked" iPhones purchased in the US.
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