Asterisk or Trixbox are fun to play with, but

there are much easier solutions. Axon on Windows is much easier and more efficient, integrating Skype too. PBXes.org is a fantastic and fully featured online pbx.

The one I like the best is MySipSwitch. If you don't have a SPA1001 but want multiple lines, this is the simplest solution and the best one too, because it does not touch audio at all and always passes CID fine, unlike PBX. You can also download it and run it on your own Windows or Linux box (what I do).