What I've understood that the low end dual SIM standby phones, I think, achieve the dual standby by using switching method. So for example in one minute, may be for 30 second time they are on one network and switches to other for rest of 30 seconds. This switching happens automatically and hence most of the time you are able to receive call and sms on both sims. However in order not to break an ongoing call, the switching is stopped, hence the not reachable tone. Further in low signal areas this continuous switching may be problematic. A dual active phone may not have these problems since it will come with two separate transceivers, but of course the battery drain will be much more. The Chinese phones had this facility and that's why they showed two signal bars, one for each sim and had two IMEI as well I think.
I welcome to be corrected.
