Skype and Vonage, I’m often asked about these 2 products. They are not the same!
Skype is a proprietary peer to peer network developed by the programmers who developed Kazza. It can be used for free PC to PC communications, and can provide excellent call quality if you assign a fixed UDP port on your firewall. It is not so good for calling landlines and mobiles as the call cost are high. In fact indirect calling providers like 1899 and 18866 are much cheaper and that is using a real phone to call another real phone! Skype is great for conference calls where everyone is on a PC. The video in Skype is a great feature. Do not buy any skype phones as these will not work with SIP. They are called VOIP phone but in reality you need to have a PC running to use them. With a SIP phone you do not need a PC. Do not use Skype unless you have a firewall otherwise Skype will use your network for other Skype users calls!
Vonage is a really expensive way of calling. You purchase a Vonage adapter and phone for not much money, and then pay them a monthly rental to make calls. They do include some free calls but it is expensive to run. It is worth buying the Vonage kit if you can unlock it and use it with a SIP provider of your choice. This is a real bargain at you can buy the kit for £20 and if you know how to unlock it, you can plug it in, configure your preferred SIP provider and make calls! Not for the faint hearted, and you loose your warrantee!
You are better of getting one of the free ADSL modems with SIP and phone lines built in and using one of the low cost SIP providers!