What is the simplest way to set up a voip structure in a Linux machine with the following features:

1) Using free software

2) For computer-to-computer calls: end-to-end encryption set by the users, unpaid, no central authority (so skype is out)

3) For computer-to-phone calls: paid or unpaid, desirable encryption on the computer side if that is even possible

4) Ability to have a number to receive calls from regular phones

My research concluded that the sip protocol is the most popular. However most discussions I've read on sip are too technical and I felt it discourages the regular user who wants to just click and talk. So I put the question above and created some separate questions about privacy with sip registrars, privacy with voip suppliers, what to look for in a sip registrar, what to look for in a voip provider.

As for the software, I noticed most software either don't provide encryption (eg Ekiga) or the software doesn't work nicely and the project is abandoned (eg Twinkle), so no option seemed satisfying.