Dual SIM
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A dual SIM mobile phone is one which has two Subscriber Identity Modules (SIMs). Dual-SIM adapters are available to allow cellphones to contain two SIMs, and to switch from one to the other as required. Some phones are designed to work with two SIMs, both of which may be active at the same time.
Dual-SIM operation allows the use of two services without the need to carry two phones at the same time. For example, the same handset can be used for business and private use with separate numbers and bills; or for travel, with an additional SIM for the country visited.
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[edit] Add-on dual-SIM adapters
Several types of adapters are available. All of them allow one SIM to be active, and to receive calls, at any given time; they provide various ways of switching from one SIM to the other. Earler versions required the phone to be switched off and on again to change from one to another. Later models allow the user to switch SIM cards via a menu or by keying in a numeric code, without switching the phone off.
The following adapters allow two SIMs to be installed in the phone. They are often described as being of a certain generation, with later generations having improved software and functionality. Some adapters require the two SIM cards to be cut to size, fitted onto a special holder and are inserted into the phone's SIM socket; this can be quite risky, since the user might end up damaging the SIM card in the process. Other adapters don't require cutting or modifying the SIM card. The adapter has two slots for the SIM cards, and a small ribbon cable, which is hooked to a PCB that mimics another SIM card. Because of the added circuitry, adapters for some phones, such as those used for older models, like the Nokia 3210 and Nokia 3310, often include thicker battery covers. Recent variations of this setup do away with the added bulk and has more miniaturized components, allowing owners of newer handsets to go to a dual SIM setup.
[edit] Dual SIM phones
Mobile phones with simultaneous dual SIM capability built in have the advantage that there are no possible legal issues, breach of contract, or voiding of warranty. They allow both SIMs to be active simultaneously and allow calls to be received on either number, unlike adapters that make one or other number active as required.
Normal GSM phones contain a single transceiver; dual-SIM phones must have dual transceivers, and need to display signal strength for both networks. A phone with a single transceiver cannot be converted to simultaneous dual-SIM operation; at most it can be made switchable with the use of an adapter.
Although battery life (talk time and standby time) of dual-SIM phones is reduced, typically by about a third, when both SIMs are active, to compensate this they often came with better capacity batteries compared to single-SIM phones.
As of Autumn 2007 some little-known Chinese companies supply inexpensive dual-SIM mobiles, mainly in Asian countries. Philips has made models Xenium 9@9w and 699 with this capability belonging to the restart to change active-line generation. Samsung also released the D880 DuoS in November 2007, a slider which can hold two SIM cards with simultaneous standby.
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