I know that the 4G network is only for data and doesn't support voice calls. Most of phones switch to 2G/3G when receiving a voice call.
But my problem is how does the phone gets to know about the reception of the voice call when it is in 4G network. We can think, a signal is sent over 4G data to the phone to switch from 4G to 2G/3G. But what will happen if the mobile data is switched off in the device, when receiving a phone call.
First things first. 4G is not only for data, it is a only -packet switched network as opposed to both circuit switched and packet switched in 3G.
Now as per your question as to how does a phone(user equipment or better UE) knows the reception of a voice call, it is done by control plane signalling over the network. So, yes, a signal is sent before the reception of call. But what is important to note that this procedure is not exclusive to 4G, 3G and lower networks also employ similar mechanism.
Now, as per your doubt if call would be received with data switched off, yes it would be received. This is because signalling messages are transferred over LTE and not data( they are two separate things). In fact event before you can access data on LTE networks a process similar to voice call is carried out, and it is technically known as data calls.
As I had mentioned in my comment, VoLTE is used to carry voice calls. This is a new standard for transmitting voice messages. The need for this method arose because LTE supports only Packet Switching, while conventionally voice was carried through Circuit Switched networks.
