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{ 1 comment… read it below or add one }
This question is like saying who is the best singer in the world, it all comes back to personal opinion. My opinion is Asterisk http://www.asterisk.org/ due to it being open source it is updated often, runs on fairly low-end hardware and has some really neat features.
Have a look at trixbox, it sets it all up for you fairly easily. http://www.trixbox.org/