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Some types of unlocked phone have two CPUs (central processing units, the part of the phone that does the 'thinking') which allow them to potentially answer two phones at the same time. Essentially it means that you can receive signals for both numbers.
Not all dual SIM card phones have two CPUs and this can have two results for consumers. If the phone has call secretary software, or your carrier has an online call secretary service then if you're on the phone and another call comes in then it'll be shipped to voicemail. If no such software or option exists on your GSM dual mobile then the other caller will just be told that your line is busy.
So can I take two phone calls at the same time on a dual SIM phone?
Again the answer to that question depends on how many CPUs your phone has, if there is more than one then there should be no problem. But if your China cellphone wholesale phone only has the one CPU then you're only going to be able to take or make one phone call at a time.
Both numbers share the same telephone list and SMS library so there's no threat of wasting hours scrolling through two different libraries to find one message.
you need a very good understanding of the transmission protocol data units concepts for that...