What is soft voip and how is it different from voip service like skype?

Apart from technical details of soft voip, I also want to know how to set it up on my home pc.

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2. Do anybody know a voip soft-phone able to manage different voip numbers at the same time?
3. How do I use Skype and another program to make free VOIP calls to landlines?
4. Is VoIP a service from ISP’s or can I just buy VoIP router and make a virtual number from the router using dsl?
5. How are Skype out and other VOIP calls routed?