The best version of asterisk so far is still Asterisk 1.4.21.2, but zaptel is not compatible with newer kernels.
http://download.vicidial.com/required-a ... ici.tar.gz is the latest version that we have had good luck with.
As far as Xen is concerned we do not recommend any level of Virtualization. Virtualization adds another layer of complexity and does not address the major factor in scaling of Asterisk. Asterisk is both a processor intensive application and a I/O intensive application. There is only one system bus in any single server. This does not change when you add Virtualization on top of it. If the I/O does not get where it is suppose to go in time, you will have problems. It is far better and less expensive to build a proper cluster with multiple smaller servers than to try and shove tons of agents onto a huge server. A quad core with between four and eight gigs of Ram is more than plenty for 25 agents dialing at a 4 to 1 ratio. You can build servers like this for less than 700 dollars using quality components if you source them well.
As for asterisk 1.8 support, don't bother. I am mostly done with it. At this point Matt needs to adjust some things in the agent interface, but he needs to finish some pay development before he can get to this. When we get there we will be looking for beta testers. Also asterisk 1.8 probably wont be very stable for production use for a long time. Support for asterisk 1.4 was added about 1.4.10 and it was not properly stable till asterisk 1.4.18 and we did not start recommending it till asterisk 1.4.20. In my tests asterisk 1.8 is not crashing, but it has some very strange errors that are limiting its scalability and causing issues with recordings.
Michael Cargile | Director of Engineering | ViciDialGroup | http://www.vicidial.com
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