If a baligh man and baligh woman agree to each other for marriage but they can't tell their parents because their elder brothers and sisters are yet to marry, can the man and girl talk to each on voice call or video call? They both are not involving any kind of adultery. They just want to talk to each other to remain in contact till their turn for marriage arrive. Only man and woman know that they agreed to marry each other and none in their families knows

If they are confident that they will not get into any love talks, the it is OK, but this is very hard to ensure. On the other hand, for how long this will last? It might take ages.
There is no rule in Islam that elder brothers and sisters have to marry first.