With some testing I noted that the phone does not receive calls through SIM1 whenever the data connection indicates H+ sign ( Data connection is though the SIM2). Yet I could receive calls when that data indicator indicated as 3g. Finally found a solution to this.
Solution: Go to Settings -> My Device -> SIM card manager and put the tick on Receive Incoming Calls, which is the first entry under General tab.
It gives a warning message saying the data speeds will be reduced by enabling this option. Still when I enabled that option, I did not see a significant reduction in speeds. I get all the calls from SIM1, yet the data connection drops whilst the phone is on a call in SIM1.
Think this works vice versa, that is Data is on SIM slot 1 and voice only SIM is on SIM slot 2.
May be this is a well known thing. I posted as this might help someone.