Where can one make a free internet phone call?

First of all you need to know a "country code" of your destination country, "city code" is also needed. And you need to know the "dialing rules" of your phone operator (as usual the country code is preceded by "+").

For international call you need to dial the whole combination of your codes:

<country code><city code><destination number>

For example if you are going to call to Sydney Opera you need to dial

+61 2 9250 7111