If a boy and a girl like each other and want to marry but they can't tell their parents due to some problem, they will tell them in future but till then is it okay if they talk to each other on voice call or video call? I was just told that they talk to each other in order to remain in contact. Is it same in that case as well?

As per the information you have given to me, it seems they're talking to each other and gaining pleasure from it therefore it is haraam. It can't be without pleasure because they have reached a conclusion they want to be with each other