Experience with Voip on Ubuntu?

[jerry]

I    had been using Sunrocket for voip phone service until they decided 
to go out of business without telling their customers.  I would like to 
know if anyone has had success with  VOIP on Ubuntu.  I   want to be 
able to call regular landline phones in the U.S. and Mexico. I don't 
need a lot of bells and whistles (3 way calling and 911 etc. ) just 
decent sound quality.  I have found a guide for voip using asterisk from 
source and freepbx but it seems that I could not call landlines with 
this setup.  Ekiga seems to have the same limitations.

Am I limited to using another commercial service? I hesitate to try 
another commercial service like Sunrocket  because they may also go out 
of business. Vonage has not been advertising like mad like they had been 
doing for quite a while. Plus I have heard a rumor the some big telco 
(Verison?) has some crucial patents and will try to stop all commercial 
voip providers.

Appreciate any information. I am using LTS 6.06 and have adsl broadband. 
I also have Feisty available.

Jerry