when you are using net to phone that is mean you are connected to the internet and using an application to dial an international number ....  then the first party will need to be connected to the internet but the second party has no need to be connected to the internet .. he just need to has a land line phone or cell phone .

When you are using VOIP you should have a good speed internet connection (over than 56 kbs) and you shouldn't be behind firewall cause VOIP need different ports of 80 or 8080 .. also if you are behind firewall you still make your calls but in this case you should use tunneling applications  ..  one of the sites that offer this service is www.net2phone.com

Finally net2phone is not a free service .. but actually you can save a lot of money when you use it instead of your phone

Add any preffered VOIP sites in the comments of you have some