Dual-SIM phone, do they use 2 GSM modules ? Anyone has high level schematic?

As far as I know from my brother who uses dual sim phone, both SIMs are active at once. It means you can receive calls on both SIMs without any special switching. Also you can receive or send messages on both SIMS. But the phone has only one GSM hardware. Only one antenna, only one transmitter etc. GSM use time divided modulation so you can run two SIMs without having two pieces of everything - that would make the phone a lot heavier than normally. (Nobody would want a lot heavier phone.) Most of the time the transmitter/receiver is turned off to save valuable power.

In the past I also saw a phone which needed to be turned off and on to switch the sims. It automatically switched the SIMs on every turn-off turn-on cycle. That was easy to use but quite inconvenient.

No, they don't have multiple GSM modules. That would drive up cost, size, weight. Since the user is only on one call or the other, there isn't a need for more than on GSM module.

EDIT Jan 31 '12. Wikipedia says there are phones that can use both at once. But I still believe there is only on GSM module.