You are not only limited to using Skype for calling 800 numbers. Many SIP services will allow you to call 800 numbers for free. You can just download any freebie softphone, or use a service like Gizmo or similar.
It's been a while since I did this but I used to use Google Talk (
Google Chat - Chat with family and friends) together with a gateway
Voice-over-IP gateway for Google Talk, MSN and Yahoo Users to make free 0800 calls in several countries. Call quality was always very good.