How does GS3 dual SIM adapter work? I have seen that some online shops sell a Samsung Galaxy S3 dual SIM adapter. I haven't found much information about how does it work, but I have doubts about it.
Some links:

https://www.amazon.it/NC2-Premium-Addatatore-Dualsim-Samsung/dp/B008E6M802/ref=sr_1_1?ie=UTF8&amp;qid=1354533958&amp;sr=8-1
http://www.ebay.it/itm/ADATTATORE-DUAL-SIM-PER-SAMSUNG-GALAXY-S3-I9300-SIII-ACE-E-HTC-ONE-2-HSDPA-/200855685432?pt=LH_DefaultDomain_101&amp;hash=item2ec3ee8938
http://www.ebay.it/itm/NC2-3G-Dualsim-Adapter-No-Cut-fur-2-Sims-im-Samsung-Galaxy-S3-/300826215660?pt=Handy_Zubeh%C3%B6r&amp;hash=item460aa3c4ec
http://www.ebay.it/itm/Adaptateur-Double-Carte-SIM-Dual-SIM-Adapter-pour-SAMSUNG-GALAXY-S3-/281033313462?pt=FR_Mobiles_PDAs_Autres&amp;hash=item416ee40cb6

In fact, I have a phone SIM card that I use for calling and receiving with my own number, and sometimes I can get a data SIM card with some free internet traffic.
The idea behind my interest for the dual SIM adapter for th GS3 is to tell the phone to call from SIM 1 and connect to UMTS from SIM 2. My simple doubt is that Android isn't simply supporting 2 cards on phones designed for 1.
I mean, if my phone had natively 2 SIM slots I could expect a menu option &quot;default call with SIM 1/2&quot;, or &quot;which SIM do you want to connect with?&quot;, but the official specs of GS3 offer 1 SIM only as standard cell phones.
The question is, then
How does a dual SIM adapter integrate with existing phone software?
Unless your phone have two SIM card slots, support as you require is not possible. The market has adapters which goes into the existing SIM card slot of phone and will help you to switch between SIMs after shutdown.  The ONLY advantage of using this instead of swapping SIMs by conventional way is to keep both SIMs together with phone and reduce wear and tear on the SIM slot.

Samsung and other manufactures have dual SIM phones which have two SIM slots and the phone can be configured accordingly for usage. Having said that, watch out for terminologies like Dual Active, Dual Standby. Unfortunately confusion prevails even in manufacturer's web sites. So, to be sure try the behaviour before you buy.

You can read about these in the Wikipedia.