Today you have many options to use for making free calls online. However, some of these options will require the person you're calling to have an account. A Skype to Skype call is free of charge. As long as both parties have a Skype account.
You can also use Messenger to make free calls online. This too requires that you both have a messenger account. Both parties must be online at the same time in order to make the call.
If you have Facetime on an iOS device you can always use this to call a person in your contact list as long as they have an iOS device.
There is an app called What's App that allows members of this app to call one another for free. Again, both parties must be on the line at the same time in order to make it work and you both must have an account.
Facebook also offers free FB calling as long as both people are online at the same time. Basically, any website or service that offers free calling will require that the person you're calling have an account and be online for the call to connect.