is "the world's first free Java
service." However, despite being Java-based, it apparently only works on Microsoft Windows
.. that's what their web page
Due to heavy use by businesses, it is difficult to make calls through Dialpad during business hours on business days, but on weekends and during the night, I have never failed to get a connection.
Once you do get through, you may notice that your conversations are a little odd; this is because while regular telephones have low latency, <100ms, between the time you speak and the time you are heard, Dialpad's latency (like that of most Voice over IP applications) is relatively high, 100-300ms. (If you call yourself using dialpad, this is especially noticable.) However, after talking for a few minutes, your awareness of this latency goes away, and the conversation feels natural.
So call up your parents! They want to hear from you, and it's free!