Thanks for taking the time to help me.
Yes I admit my mistake , VOIP is definitely a general term for the protocol. No, I didn't mean the intricacies of the software.
I will tell you my problem. I was using SmartVoip software on my PC laptop for making calls from my laptop to mobile and landline phones worldwide. I tried to use Telephone 0.14.3 on my powerbook but after downloading and installing clicking on the icon doen't do anything. Then I came to know that the version of this software needs operating system leater than OS 10.5, which powerbook G4 cannot run.
Another of my friends who also has OS 10.4.11, is using Express talk for making phone calls from his computer( macbook ). When I tried to run this software on my machine, it gave a message which said that this software cannot run on this machine.
I googled for any software for VOIP on mac, and got this url.
http://www.pure-mac.com/voip.html
Out of the software given in this, I have tried telephone already without success. I am yet to try Gizmo5. Skype is my last option because I used to get my computer very slow running it, particularly the page opened and closed very very slowly.
Has anybody tried Gizmo5.
Sorry I am new to Mac so please forgive my ignorance. Because of this software issue I packed my powerbook and started using the old PC laptop. Will let it be known if my experiment with Gizmo5.4 is successful.