Bitcoin is a virtual and unregulated digital currency, not issued by any central bank. It has no central authority and it can be exchanged like currency notes. It maintains anonymity. It's price is $11,182.50 as of June 25th 2019. Know more about bitcoins here
NFS(Network File System) is developed by Sun Microsystems for sharing files/folders between Unix systems. It allows to mount local file system on a network or a remote host to interact.
IMO Messenger app is one of the prominent video calling apps which enables you to chat with people miles away. It would be the better option without any odds like overcharge and loss of signals. The best thing is - It works well in the weak signals and it’s free. More about it here