Thanks for the suggestion Wilcre. I'll try it to see if it helps. In the meanwhile I tried swapping my wife's iPhone 4 SIM with HOX+ and I was able to receive calls without any problem. After that I tried doing a factory restore/reset and looks like the problem is gone even with my original SIM. The phone is able to switch back to 4G from 4G LTE on receiving a call and answer. I'll see if it continues to work tomorrow and post back if I see this issue again.