Easy voice talk between 2 computers in 2 countries?

My plan is to enable 2 sets of unsophisticated users (children and their grandparents) in 2 different countries to talk easily. In practice I see it as: 1. A quiet Linux computer is on all the time in both places. 2. The user at one computer should be able to switch to an already running application and call the other party in the other country. 3. The computer in the other country should ring. 4. The person in the other country hearing the ring, should be able to answer the call. Again - nothing more complicated than pressing a button in an application. 5. Video option is a bonus but not a must. I unfortunately have little experience with VoIP. But I could do a fair job setting up firewalls as it might be necessary in such scenario. I am also used to various Linux distros. The question really is what applications could fit my purpose and if I must run a server (voip?) in one of the countries. As for users - less choice is better, if I could I would use this just for communication between these 2 "computer stations". Maybe even with telephone headsets :) Many thanks for your suggestions.