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|This article relies on references to primary sources. (March 2011)|
A Dual SIM mobile phone is one which holds two SIM cards. Initially, dual-SIM adapters were made available to use in regular mobile phones to allow them to contain two SIMs, and to switch from one to the other as required. This combination is called a standby dual-SIM phone. The first Dual SIM mobile phone was Benefon Twin manufactured by Benefon in Finland. It was launched on the market in 2000.
More recently, some phones have been produced that can natively work with two SIMs, both of which may be active at the same time. These are active dual-SIM phones. There are also some Chinese triple-SIM phones.
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. Using multiple SIM cards allows the user to take advantage of different pricing plans for calls and text messages to certain destinations as well as mobile data usage.
These phones have until recent years been largely eschewed by the larger phone manufacturers partly due to their close ties with mobile phone networks who would prefer that customers use only one network exclusively. However more recently manufacturers such as Nokia and Samsung have started producing these phones, which will target customers mostly from developing countries.
- Pseudo-Dual - for convenience to use second SIM (as rule, it has «no major» slot for this) without both SIMs connected with owner's mobile service providers (MSP).
- Dual SIM Shift - basic phones which have 2 SIM Card Slots, only one SIM is active (able to receive calls), and is shown on the screen, the users may switch to another SIM by choosing from the menu.
- Dual SIM Stand-by (DSS) - allows two SIM cards to be on standby waiting for a call. When a call is established on one SIM card, the other is no longer active. Callers to the other SIM will hear a message that the phone is switched off or they'll be redirected to voicemail.
- Dual SIM Active (DSA) - allows the device to be connected to both networks at the same time. The device can switch between two calls without dropping either, so while you are on a call you can still receive calls on the other number. These devices need to have two transceivers. Nowadays these phones are not manufactured by the big brand names, because it's said to be hard not to exceed acceptable SAR (Specific absorption rate) levels in a single device.
- Dual SIM Dual Call/Dual talk - Both SIMs are able to receive calls, once active call on one SIM is established, a call can still be received on the other SIM. If user likes he can switch between two calls without disconnecting any of them. This is mostly used in China. These phones have 2 RFs, 2 SIMs and 2 Modem Stacks. Also known as DSDA (Dual SIM Dual Active), Dual SIM Active or Duos. (LG GX500)
When one SIM has PS Data active call, other SIM can still get voice (CS) calls.
Chinese Dual SIM phones 
There are many dual SIM phones now available from the Chinese market. Due to comfort and usability they are getting more and more popular for both: business and private use, all over the world. The phones, which also usually include touch screen interfaces and other modern features, typically retail for a much lower price than branded models. Due to the extra battery usage of the additional sim slot it can happen the phones can be supplied with two battery packs also. Popular in Asia, these inexpensive dual SIM mobile phones are now also available to buy directly from Europe via eBay or internet sites, with reports that all slots are compatible with the major EU mobile networks.
Active Dual SIM phones 
Mobile phones with built-in simultaneous dual SIM capability allow both SIMs to be active simultaneously and allow calls to be received on either number at any given time. Most such phones have two transceivers built in one of which may support 2G and 3G while another may only support 2G.
Though a new generation of Dual SIM Phones, called "Dual SIM Dual Standby" (DSDS) provide the ability to have 2 active SIM simultaneously, using only one transceiver. This is for example technology used in Freeyourmobile M180 Dual SIM Mobile Phone, K-Touch A982, LG-GX200, Philips Xenium - X100, Samsung B7722, Samsung B5722 and Samsung E2152.
Battery life (talk time and standby time) of active dual-SIM mobile phones is reduced, typically by about a third. For instance, Device manufacturer Samsung declares a standby time for its Champ Duos E2652 Dual SIM phone of 476 hours when used with one SIM card and of 322 hours when used with two SIM cards. To compensate the shorter battery life, some Dual SIM phones ship with better capacity batteries, or in the case of Chinese-made phones, bundle their units with two batteries.
In November 2007, Verzio — a Singapore-based 3C (computing, communications, consumer electronics) brand launched the world's first 3G Dual-Triband dual-SIM model named Duplii. The same brand also launched the Twinn, 2.75G dual-SIM model. Powered by 2 processors, the Duplii and Twinn provide real dual-SIM real dual-connecting capability.
There are some little-known Chinese companies supplying inexpensive dual-SIM mobiles, mainly in Asian countries. The latest models includes W006 WiFi Dual Sim Mobile Phone, The M60 - Quadband Dual SIM Touchscreen mobile phone was launched in April 2010 being the 1st of the Chinese touch screen only mobiles with no keypad, including many features WiFi, Dual IMEI, Media playback & camera etc. to rival normal mobile phones.
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 and the D780 which came much later both have limited function on the 2nd SIM card, and there is a Windows Mobile based dual-SIM phone too, called E-TEN glofiish DX900 (released in November 2008, been renamed to Acer Tempo DX900 in January 2009, after Acer bought E-TEN).
Popular Indian mobile companies such as Karbonn Mobiles, Micromax Mobile, Spice, Lemon Mobiles, LAVA Mobiles launch many feature phones with Dual SIM technology. Even few smart phones from these companies are Dual SIM based.
Nokia has launched two dual SIM phone models C2-00 and X1-01. Both are 2G phones. Also, Nokia will launch more dual SIM devices such as C2-03, C2-06 featuring a 'Touch and Type' user interface with 'Easy-Swap SIM' facility. All of the models produced so far are Dual-SIM Standby phones - they have only 1 radio. Nokia at this point (January 2013) has no publicly announced plans to produce any Dual-SIM Active phones. Also all current Nokia Dual-SIM phones use the Series 40 interface.
Samsung "Dual SIM Always on" feature 
Samsung in their marketing materials has introduced a term „Dual SIM Always on”. The term is misleading, since it doesn’t mean what is says – both SIM cards are not always on. All phones with this feature are regular Dual SIM Stand-by (DSS) phones with 1 transceiver (radio) – 2nd SIM is disconnected when a call is in progress on SIM 1 and vice versa.
This includes GALAXY Ace DUOS (GT-S6802), GALAXY S Duos (GT-S7562), Galaxy Grand Duos (GT-I9082) and other currently (May 2013) manufactured Android phones by Samsung – they all are Dual SIM Stand-by (DSS) phones.
Manual for these phones states: “Your device supports dual standby with two different networks. You cannot make or answer calls on both networks at the same time.”
These Android phones have a menu option “Dual SIM Always on” which when activated activates call forwarding on the carrier’s network. This can be done manually on any phone, regardless of manufacturer, e.g. enable call forwarding to SIM 1 when SIM 2 is disconnected. Call Forwarding must be provided by the carrier, often for a fee, subscriber will also be charged for call forwarding on a minute by minute basis, this depends on the subscription agreement.
See also 
|Wikimedia Commons has media related to: Dual SIM|
- Dual mode mobile
- Subscriber Identity Module
- SIM cloning
- International Mobile Equipment Identity
- "Dual SIM Nokia C2 revealed". Nokia Corporation. Retrieved 18 August 2010.
- "Nokia C1 unleashed with double-SIM functionality". Nokia Corporation. Retrieved 18 August 2010.